cols4all: introduction

Introduction

Color is essential in many charts and maps. There are a lot of color palettes to choose from: most visualization software tools have their own palettes and there are many other series of color palettes, such as ColorBrewer (Harrower and Brewer, 2003). To make life easier for R users, there are a couple of packages that contain a large collection of palettes, most notably pals (Wright, 2021) with 139 and paletteer (Hvitfeldt, 2021) with 2569 (!) palettes. However, people often cannot see the trees through the forest, so therefore they tend to stick with the color palettes they know, or with the most popular ones.

The cols4all package also contains a large collection of palettes (to be precise 436 at the time of writing), but with the central question: which palettes are good and why? There is no simple answer, since there are many aspects to take into account, which may have opposite effects. In cols4all we examine the following aspects:

Color Blind Friendliness First and most importantly, is a palette suitable for colorblind people? About 5% of all humans have a color vision deficiency, and we want to include them; hence the package name “colors for all”.
HCL Analysis Does a palette work well for statistical purposes? We analyse this with the HCL (hue-chroma-luminance) color space. Concrete questions for these analysis are the following. How vivid are colors? Do the colors stand out about equally (which we call “fairness”)? What hue ranges are used for quantitative color palettes?

Contrast What is the contrast between the colors in the palette? We focus on two specific questions: 1) How well is text readable (also for people with a visual impairment)? 2) Are border lines needed to separate colors?
3D Blues Does the palette contain blue colors that could result in a floating effect?

Currently, cols4all contains palettes from several popular and lesser known color palette series: “brewer”, “c4a”, “carto”, “hcl”, “kovesi”, “met”, “parks”, “poly”, “scico”, “seaborn”, “stevens”, “tableau”, “tol”, “viridis” , and “wes”. Stand-alone palettes that are included have been put in the series “misc” (miscellaneous). Own palettes series can be added as well.

Color palettes are organized and made consistent with each other. For instance, all sequential palettes go from light to dark. Furthermore, for each color palette a color for missing values is assigned, which is especially important for spatial data visualization. Currently we support several types: categorical (qualitative) palettes, sequential palettes, diverging palettes, and bivariate palettes (divided into four subtypes).

The figure above shows the main tool of this package, a dashboard. The first tab page contains an overview table of all palettes (given the input selection criteria). The other tab pages are for further palette-wise analyses.

Color palettes also are important in areas other than data visualization, such as arts, web design, and fashion. The same aspects apply as well here, but obviously different criteria apply: the colors of a dress don’t have to be distinguished by colorblind people, websites often contain colors that purposely stand out more than others, and there are paintings that purposely use visual illusions.

Related R packages

The foundation of this package is another R package: colorspace. We use this package to analyse colors. For this purpose and specifically for color blind friendliness checks, we additionally use colorblindcheck.

There are a few other pacakges with a large collection of color palettes, in particular pals and paletteer. There are a few features that distinguishes cols4all from those packages:

Color palettes are characterized and analysed. Properties such as color blindness, fairness (whether colors stand out about equally), and contrast are determined for each palette.
Bivariate color palettes are available (besides the three main palette types: categorical, sequential, and diverging).
Own color palettes can be loaded and analysed.
Color for missing values are made explicit.
Palettes are made consistent with each other to enable comparison. For instance, black and white are (by default) removed from categorical palettes. Another standard that we adapt to is that all sequential palettes go from light to dark and not the other way round.
There is native support for ggplot2 and tmap (as of the upcoming version 4).
There are a couple of exporting options, including (bibtex) citation.

Using cols4all

Installation

The cols4all is available on CRAN and can be installed as follows

install.packages("cols4all", dependencies = TRUE)

The latest development version can be installed as follows:

remotes::install_github("mtennekes/cols4all", dependencies = TRUE)

Getting started

Load the package:

library(cols4all)

The main tool is a dashboard (shown above), which is started with:

c4a_gui()

What types and series are available?

c4a_types()
#>   type                          description
#> 1  cat                          categorical
#> 2  seq                           sequential
#> 3  div                            diverging
#> 4 bivs  bivariate (sequential x sequential)
#> 5 bivc bivariate (sequential x categorical)
#> 6 bivd   bivariate (sequential x diverging)
#> 7 bivg bivariate (sequential x desaturated)

c4a_series()
#>     series                                         description
#> 1   brewer                                ColorBrewer palettes
#> 2      c4a                  cols4all palettes (in development)
#> 3    carto                          Palettes designed by CARTO
#> 4      hcl  Palettes from the Hue Chroma Luminance color space
#> 5   kovesi                   Palettes designed by Peter Kovesi
#> 6      met Palettes inspired by The Metropolitan Museum of Art
#> 7     misc                              Miscellaneous palettes
#> 8    parks                 Palettes inspired by National Parks
#> 9     poly               Qualitative palettes with many colors
#> 10   scico     Scientific colour map palettes by Fabio Crameri
#> 11 seaborn            Palettes from the Python library Seaborn
#> 12 stevens                Bivariate palettes by Joshua Stevens
#> 13 tableau                        Palettes designed by Tableau
#> 14     tol                       Palettes designed by Paul Tol
#> 15 viridis          Palettes fom the Python library matplotlib
#> 16     wes                   Palettes from Wes Anderson movies

How many palettes per type x series?

c4a_overview()
#>         cat seq div bivs bivc bivd bivg
#> brewer    8  18   9    2    1    1   NA
#> c4a      NA  NA   2    2   NA    2    5
#> carto     6  21   7   NA   NA   NA   NA
#> hcl       9  23  11   NA   NA   NA   NA
#> kovesi   NA  17  13   NA   NA   NA   NA
#> met      33   8  14   NA    1   NA   NA
#> misc      5  NA  NA   NA    3   NA   NA
#> parks    22   5   3   NA   NA   NA   NA
#> poly      9  NA  NA   NA   NA   NA   NA
#> scico    NA  18  14   NA    2   NA    1
#> seaborn   6   4   2   NA   NA   NA   NA
#> stevens  NA  NA  NA    5   NA   NA   NA
#> tableau  29  23  28   NA   NA   NA   NA
#> tol       7   7   3   NA   NA   NA   NA
#> viridis  NA   7   1   NA   NA   NA   NA
#> wes      18  NA   1   NA   NA   NA   NA

What palettes are available, e.g diverging from the hcl series?

# Diverging palettes from the 'hcl' series
c4a_palettes(type = "div", series = "hcl")
#>  [1] "hcl.blue_red1"    "hcl.blue_red2"    "hcl.blue_red3"    "hcl.red_green"   
#>  [5] "hcl.purple_green" "hcl.purple_brown" "hcl.green_brown"  "hcl.blue_yellow2"
#>  [9] "hcl.blue_yellow3" "hcl.green_orange" "hcl.cyan_magenta"

Give me the colors!

# select purple green palette from the hcl series:
c4a("hcl.purple_green", 11)
#>  [1] "#492050" "#82498C" "#B574C2" "#D2A9DB" "#E8D4ED" "#F1F1F1" "#C8E1C9"
#>  [8] "#91C392" "#4E9D4F" "#256C26" "#023903"

# get the associated color for missing values
c4a_na("hcl.purple_green")
#> [1] "#868686"

Plot these colors:

c4a_plot("hcl.purple_green", 11, include.na = TRUE)

Using cols4all palettes in ggplot2

library(ggplot2)
data("diamonds")
diam_exp = diamonds[diamonds$price >= 15000, ]

# discrete categorical scale
ggplot(diam_exp, aes(x = carat, y = price, color = color)) +
    geom_point(size = 2) +
    scale_color_discrete_c4a_cat("carto.safe") +
    theme_light()


# continuous diverging scale
ggplot(diam_exp, aes(x = carat, y = depth, color = price)) +
    geom_point(size = 2) +
    scale_color_continuous_c4a_div("wes.zissou1", mid = mean(diam_exp$price)) +
    theme_light()

Overview of functions

Main functions:

c4a_gui Dashboard for analyzing the palettes
c4a Get the colors from a palette (c4a_na for the associated color for missing values)
c4a_plot Plot a color palette

Palette names and properties:

c4a_palettes Get available palette names
c4a_series Get available series names
c4a_types Get implemented types
c4a_overview Get an overview of palettes per series x type.
c4a_citation Show how to cite palettes (with bibtex code).
c4a_info Get information from a palette, such as type and maximum number of colors
.P Environment via which palette names can be browsed with auto-completion (using $)

Importing and exporting palettes:

c4a_data Build color palette data
c4a_load Load color palette data
c4a_sysdata_import Import system data
c4a_sysdata_export Export system data

ggplot2

scale_<aesthetic>_<mapping>_c4a_<type> e.g. scale_color_continuous_c4a_div Add scale to ggplot2.

Color spaces

Visual perception, in particular colors, is complex. For the purpose of information visualization it is explained in an accessible way by Ware (2019). In this vignette, we briefly describe the color spaces that are needed to understand the functionality of cols4all.

The human eye has three types of cones (specific photoreceptor cells in the retina), which are sensitive to light of different wavelengths: long, medium, and short. Their peak sensitivities (of 565, 540, and 440nm respectively) correspond to the primary colors red, green, and blue respectively. In computer graphics, a color is usually defined as RGB, which is a mixture of those three primary colors. RGB colors are often represented in hex-format (e.g. #FF00FF), where the first two characters specify red, the third and fourth green and the last two blue. Therefore #FF00FF is purple. A standard RGB space in computer graphics is sRGB (which exactly defines the primary colors, the white point, and a so-called gamma correction which is needed to perceive a grayscale linearly).

To analyse color palettes, we also use the polarLUV color space, which is a transformation of the CIELUV space. In this space, known as HCL, a color is defined by three variables:

Hue The hue of a color is how we call casually name colors: e.g. yellow, purple, blue, orange, etc. It is defined as a polar coordinate, so takes values from 0 to 360 degrees.
Chroma The vividness or intensity of the color. The higher, the more colorful. The maximum chroma depends on the hue. Grayscale colors have 0 chroma. The maximum value depends the hue and luminance.
Luminance The amount of light emitted from an object (e.g. a computer screen). It is similar to brightness, although the latter is a relative measure. It takes values from 0 to 100.

Indicators

Colorblind friendliness

First of all, we use the distance measure between two colors A and B from CIE as defined in 2000 (see Lindbloom, Bruce Justin. Delta E (CIE 2000).).

For simplicity, we apply two labels: colorblind friendly ☺ and colorblind unfriendly 👀. A palette without such label is classified in between. Our definition of these labels depend on the palette type:

(categorical) The minimum distance metric (min_dist) is the minimum distance between any two colors for any color vision deficiency type.
- ☺ (colorblind friendly): min_dist >= 10
- 👀 (colorblind unfriendly): min_dist <= 2
  
  These numbers are parameters that can be changed via c4a_options.
(sequential) The minimum step metric (min_step) is used, which is the smallest distance between any two adjacent colors (for any color vision deficiency type).
- ☺ (colorblind friendly): min_step >= 5
- 👀 (colorblind unfriendly): min_dist <= 1
  
  Again, these numbers are parameters that can be changed via c4a_options. Note that this metric is not very useful when a continuous scale is used. In that case, it is recommended to analyse the palette with say 7 or 9 colors.
(diverging) The min_step metric used in a similar way as for sequential palettes. In addition, the inter wing distance metric (inter_wing_dist) is used, which is the minimum color distance between any color in the left wing to any color in the right wing of the palette (for any color vision deficiency type).
- ☺ (colorblind friendly): min_step >= 5 and inter_wing_dist >= 10
- 👀 (colorblind unfriendly): min_dist <= 1 or inter_wing_dist <= 4
  
  These parameters can be changed via c4a_options.
(bivariate (seq x seq)) Three sequential palettes are extracted: a) the first column, b) the bottom row, and c) the diagonal. These are combined to form three diverging palettes: ab, ac, and bc. For each of these diverging palette, the same checks are applied as described above for regular diverging palettes.
(bivariate (seq x cat)) For each row, the same checks apply as for a categorical palette
(bivariate (seq x div)) Three sequential palettes are extracted: a) the first column, b) the middle column column, and c) the last column. These are combined to form three diverging palettes: ab, ac, and bc. For each of these diverging palette, the same checks are applied as described above for regular diverging palettes.
(bivariate (seq x desaturated)) A diverging palette is formed from the first and the last column. The same checks are applied as described above for regular diverging palettes.

HCL analysis

Regarding the used hues, there is no simple good and bad. However, for quantitative palettes we distinguish

(sequential)
- Single hue palettes (🖌) which are recommended for quantitative analysis, but harder to distinguish colors
- Spectral (🌈) which are easy to distinguish colors, but less suitable for quantitative analysis
(diverging)
- Two-hue palettes (☯) each wing has its own distinct hue, which is highly recommended
- Spectral (🌈) which are easy to distinguish colors, but less suitable for quantitative analysis

Regarding the used chroma / vividness, we label a color palette

vivid (🕶) if it contains a color with a chroma value of 100 or higher
pastel (❉) if all colors have chroma values of at most 70

These threshold numbers can be configured via options (to do).

Pastel (low chroma) colors are recommended for space-filling visualizations, like maps and bar charts. Vivid (high chroma) colors for small objects, such as dots, lines, and text labels.

We also use the chroma and luminance values of palette colors to find out whether some colors stand out more than other colors. Ideally, a categorical palette should contain colors that stand out about equally (for otherwise, one color will draw more attention than another, which may bias our perception and interpretation of the shown data). Colors with a high chroma value stand out more than less chromatic colors. Furthermore, against a bright background, dark colors (low luminance) stand out more, while bright colors (high luminance) stand out more against a dark background. Luminance is only considered for categorical palettes, because quantitative palettes (sequential and diverging) often map a numeric variable to luminance.

Let us define the luminance range (Lrange) as the maximum minus the minimum luminance value of the palette and the chroma range (Crange) as the maximum chroma value minus the minimum chroma value.

Regarding Fairness we consider two metrics: the luminance range (Lrange) and the chroma range (Crange). We call a palette:

fair (♡) if Crange <= 50 and (for categorical palettes only) Lrange <= 30
unfair (⨯) if Crange <= 80 or (for categorical palettes only) Lrange >= 50

Note that harmonic color palettes are usually not color blind friendly. Furthermore, when the luminance values of the colors are about equal, the contrast of those colors is low, which requires the use of border lines (see below).

Contrast

The border between two colored shapes appears wobbly when the colors are equally luminant (bright), no matter what hue (red, blue, etc.) they have. This visual illusion is called equiluminance.

The contrast ratio is a measure for equiluminance, calculated as CR = (L1 + 0.05) / (L2 + 0.05), where L1 and L2 are the luminances (normalized between 0 and 1) of the lighter and darker colors, respectively. Note that the minimum contrast ratio is 1 and the maximum 21.

In the overview table, there are three flags that indicate low contrast (CR < 1.2):

🏁: Between two colors of the palettes
🏳: Between one color of the palette and white
🏴: Between one color of the palette and black

The go-to solution to prevent wobbly borders is by using black or white (depending of the lightness of the colors) border lines.

3D Blues

Pure blue colors, by which we mean a 0 or very low number for the R (red) and G (green) channels and a high number for the B (blue) channel may cause a visual illusion called chromostereopsis. This effect is especially prominent when blue objects are shown next to red objects against a black background.

The table column “3D Blues” indicates when a palette contains a pure blue color. The used symbol is B7#9 (which refers to a music chord used in blues music).