R Colors: Names, RGB/HSV, and Building a Color Chart
efg, Stowers Institute for Medical Research (original script); annotated version
2026-07-19
1 Background
R ships with a built-in palette of named colors, returned by
colors(). As of this writing (and for a long time now) it
holds 657 names — things like "steelblue",
"tomato3", or "gray42". This document walks
through a classic script by efg (Stowers Institute) that turns that list
into a printable reference chart, and along the way explains the
underlying R color machinery: how names map to RGB, how RGB maps to HSV,
and how to pick readable text/background combinations automatically.
The original script writes everything to a file called
COLOR_CHART.pdf. Here we reproduce each piece as an inline,
explained chunk so it renders directly in this document; the full
original (PDF-producing) script is included as an appendix at the end
for reference.
# The chart layout below hard-codes 25 columns x 27 rows = 675 slots for 657 colors.
# This assumes the palette size hasn't changed between R versions.
length(colors())## [1] 6572 How R represents colors
R gives you three interchangeable ways to specify a color:
- By name — a string from
colors(), e.g."firebrick". - By hex code —
"#FF0000", the familiar web-style RRGGBB (optionally with alpha, RRGGBBAA). - By numeric index — a legacy palette
(
palette()), rarely used now.
col2rgb() converts any of the first two into the
underlying 0–255 RGB triplet:
col2rgb("yellow")## [,1]
## red 255
## green 255
## blue 0col2rgb("steelblue")## [,1]
## red 70
## green 130
## blue 180RGB is how the color is stored, but it is not how humans reason about
color. For that, R (via rgb2hsv()) can convert to
HSV — Hue, Saturation, Value — which separates what
color (hue, as an angle 0–1 representing 0–360°) from how
vivid (saturation) and how bright (value):
rgb2hsv(col2rgb("steelblue"))## [,1]
## h 0.5757576
## s 0.6111111
## v 0.7058824Sorting colors by hue-then-saturation-then-value is what makes the
second chart below (“sorted by hue”) group visually similar colors
together, instead of the arbitrary alphabetical-ish order
colors() returns them in.
3 Choosing readable text color: a contrast heuristic
Rectangles need a label (the color’s name and index) drawn on top
of them. White text on "yellow" is unreadable; black
text on "navy" is unreadable. The script uses a cheap
luminance heuristic: average the R, G, and B channels, and if that
average is above the midpoint (127.5), the background is “light” so use
black text; otherwise use white text.
SetTextContrastColor <- function(color) {
ifelse(mean(col2rgb(color)) > 127, "black", "white")
}
SetTextContrastColor("white")## [1] "black"SetTextContrastColor("black")## [1] "white"SetTextContrastColor("red")## [1] "white"SetTextContrastColor("yellow")## [1] "black"This is a simplification — perceptual luminance weights green much
more heavily than blue (roughly
0.299*R + 0.587*G + 0.114*B), so a plain channel average
can misjudge some saturated blues or reds. It’s good enough for a quick
reference chart, though, which is the point here.
We precompute the contrast color for every entry in
colors() once, so the plotting loops below can just look it
up instead of recomputing it per rectangle:
TextContrastColor <- unlist(lapply(colors(), SetTextContrastColor))
table(TextContrastColor)## TextContrastColor
## black white
## 416 2414 Chart 1a: colors in index order
The simplest chart just walks colors() in the order R
returns it, laying out 25 colors per row. Each rectangle is drawn with
rect(), positioned by its row/column, filled with the color
itself, and labeled with its numeric index
(SetTextContrastColor decides whether that label is black
or white).
colCount <- 25 # number of rectangles per row
rowCount <- ceiling(length(colors()) / colCount)
oldpar <- par(mar = c(1, 1, 2, 1))
plot(c(1, colCount), c(0, rowCount), type = "n", ylab = "", xlab = "",
axes = FALSE, ylim = c(rowCount, 0))
title("R colors (index order)")
for (j in 0:(rowCount - 1)) {
base <- j * colCount
remaining <- length(colors()) - base
RowSize <- ifelse(remaining < colCount, remaining, colCount)
rect((1:RowSize) - 0.5, j - 0.5, (1:RowSize) + 0.5, j + 0.5,
border = "black", col = colors()[base + (1:RowSize)])
text((1:RowSize), j, paste(base + (1:RowSize)), cex = 0.7,
col = TextContrastColor[base + (1:RowSize)])
}
par(oldpar)Notice how little visual structure there is: R’s
colors() order is alphabetical by name (with numbered
variants like "blue1".."blue4" interleaved),
not by appearance, so neighboring cells jump around the color wheel.
5 Chart 1b: colors sorted by hue, saturation, value
To get a chart that actually reads as a gradient, we convert every
color to HSV and sort by hue first, then saturation, then value.
order() gives us the permutation of indices that achieves
that sort, and we index into colors() with it.
RGBColors <- col2rgb(colors())
HSVColors <- rgb2hsv(RGBColors[1, ], RGBColors[2, ], RGBColors[3, ], maxColorValue = 255)
HueOrder <- order(HSVColors[1, ], HSVColors[2, ], HSVColors[3, ])
oldpar <- par(mar = c(1, 1, 2, 1))
plot(0, type = "n", ylab = "", xlab = "", axes = FALSE,
ylim = c(rowCount, 0), xlim = c(1, colCount))
title("R colors -- sorted by hue, saturation, value")
for (j in 0:(rowCount - 1)) {
for (i in 1:colCount) {
k <- j * colCount + i
if (k <= length(colors())) {
rect(i - 0.5, j - 0.5, i + 0.5, j + 0.5,
border = "black", col = colors()[HueOrder[k]])
text(i, j, paste(HueOrder[k]), cex = 0.7,
col = TextContrastColor[HueOrder[k]])
}
}
}
par(oldpar)The label in each cell is still the original index into
colors() — so you can look up a color visually here, then
use that number (or better, colors()[index]) to reference
it by name in code.
6 Getting a color’s hex and decimal RGB
For a reference sheet you usually want the exact hex code (for
CSS/HTML/design tools) alongside the decimal RGB triplet (for R/plotting
code). sprintf() builds both from col2rgb()’s
output in one line:
GetColorHexAndDecimal <- function(color) {
c <- col2rgb(color)
sprintf("#%02X%02X%02X %3d %3d %3d", c[1], c[2], c[3], c[1], c[2], c[3])
}
GetColorHexAndDecimal("yellow")## [1] "#FFFF00 255 255 0"GetColorHexAndDecimal("steelblue")## [1] "#4682B4 70 130 180"7 Chart 2: paginated detail sheet (name + hex + decimal)
The final part of the original script produces a multi-page listing: two columns of 50 colors each per page, each row showing a filled swatch, the color’s index, its name, and its hex/decimal RGB (in a monospace font so the numbers align). Below is a single representative page reproduced inline; the appendix has the full paging loop that generates every page (this is what the original script writes across the multi-page PDF).
index <- paste(1:length(colors()))
HexAndDec <- unlist(lapply(colors(), GetColorHexAndDecimal))
PerColumn <- 50
PerPage <- 2 * PerColumn
page <- 1
oldpar <- par(mar = c(1, 1, 1, 1))
plot(0, type = "n", ylab = "", xlab = "", axes = FALSE,
ylim = c(PerColumn, 0), xlim = c(0, 1))
title("R colors")
mtext(paste("page", page), side = 1, adj = 1, line = -1)
base <- PerPage * (page - 1)
# Column 1
remaining <- length(colors()) - base
ColumnSize <- ifelse(remaining < PerColumn, remaining, PerColumn)
rect(0.00, 0:(ColumnSize - 1), 0.49, 1:ColumnSize,
border = "black", col = colors()[(base + 1):(base + ColumnSize)])
text(0.045, 0.45 + (0:(ColumnSize - 1)), adj = 1, index[(base + 1):(base + ColumnSize)],
cex = 0.6, col = TextContrastColor[(base + 1):(base + ColumnSize)])
text(0.06, 0.45 + (0:(ColumnSize - 1)), adj = 0, colors()[(base + 1):(base + ColumnSize)],
cex = 0.6, col = TextContrastColor[(base + 1):(base + ColumnSize)])
save <- par(family = "mono")
text(0.25, 0.45 + (0:(ColumnSize - 1)), adj = 0, HexAndDec[(base + 1):(base + ColumnSize)],
cex = 0.6, col = TextContrastColor[(base + 1):(base + ColumnSize)])
par(save)
# Column 2
if (remaining > PerColumn) {
remaining2 <- remaining - PerColumn
ColumnSize2 <- ifelse(remaining2 < PerColumn, remaining2, PerColumn)
rect(0.51, 0:(ColumnSize2 - 1), 1.00, 1:ColumnSize2,
border = "black", col = colors()[(base + PerColumn + 1):(base + PerColumn + ColumnSize2)])
text(0.545, 0.45 + (0:(ColumnSize2 - 1)), adj = 1,
index[(base + PerColumn + 1):(base + PerColumn + ColumnSize2)],
cex = 0.6, col = TextContrastColor[(base + PerColumn + 1):(base + PerColumn + ColumnSize2)])
text(0.56, 0.45 + (0:(ColumnSize2 - 1)), adj = 0,
colors()[(base + PerColumn + 1):(base + PerColumn + ColumnSize2)],
cex = 0.6, col = TextContrastColor[(base + PerColumn + 1):(base + PerColumn + ColumnSize2)])
save <- par(family = "mono")
text(0.75, 0.45 + (0:(ColumnSize2 - 1)), adj = 0,
HexAndDec[(base + PerColumn + 1):(base + PerColumn + ColumnSize2)],
cex = 0.6, col = TextContrastColor[(base + PerColumn + 1):(base + PerColumn + ColumnSize2)])
par(save)
}
par(oldpar)To generate every page (not just page 1) and get the full
multi-page reference sheet, wrap that same block in a loop over
page in 1:ceiling(length(colors()) / PerPage) and, when
knitting, either emit one PDF page per iteration or (for HTML) let each
iteration produce its own plot — see the appendix for the loop as
written in the original script.
8 Takeaways
colors()is R’s built-in named palette (657 entries);col2rgb()andrgb2hsv()convert between name/hex, RGB, and HSV representations.- Sorting by HSV hue (not the default alphabetical order) is what makes a color chart visually navigable.
- A simple average-of-channels test is a cheap-but-imperfect way to auto-pick readable black/white text on an arbitrary background color.
sprintf("#%02X%02X%02X ...")is the idiomatic way to format an RGB triplet as both hex and decimal in one string.
9 Appendix: original
script (writes COLOR_CHART.pdf)
# efg, Stowers Institute for Medical Research
# efg's Research Notes: http://research.stowers-institute.org/efg/R/Color/Chart 6 July 2004. Modified 23 May 2005.
pdf("COLOR_CHART.pdf", width = 10, height = 10)
# save to reset at end
oldparameters <- par(mar = c(1, 1, 2, 1), mfrow = c(2, 1))
# Be cautious in case definition of "colors" changes. Use some hard-coded constants since this is not expected to change.
stopifnot(length(colors()) == 657)
# 0. Setup
SetTextContrastColor <- function(color) {ifelse(mean(col2rgb(color)) > 127, "black", "white")}
TextContrastColor <- unlist(lapply(colors(), SetTextContrastColor))
# 1a. Plot matrix of R colors, in index order, 25 per row.
colCount <- 25
rowCount <- 27
plot(c(1, colCount), c(0, rowCount), type = "n", ylab = "", xlab = "", axes = FALSE, ylim = c(rowCount, 0))
title("R colors")
mtext("http://research.stowers-institute.org/efg/R/Color/Chart", cex = 0.6)
for (j in 0:(rowCount - 1)) {
base <- j * colCount
remaining <- length(colors()) - base
RowSize <- ifelse(remaining < colCount, remaining, colCount)
rect((1:RowSize) - 0.5, j - 0.5, (1:RowSize) + 0.5, j + 0.5, border = "black", col = colors()[base + (1:RowSize)])
text((1:RowSize), j, paste(base + (1:RowSize)), cex = 0.7, col = TextContrastColor[base + (1:RowSize)])
}
# 1b. Plot matrix of R colors, in "hue" order, 25 per row.
RGBColors <- col2rgb(colors()[1:length(colors())])
HSVColors <- rgb2hsv(RGBColors[1, ], RGBColors[2, ], RGBColors[3, ], maxColorValue = 255)
HueOrder <- order(HSVColors[1, ], HSVColors[2, ], HSVColors[3, ])
plot(0, type = "n", ylab = "", xlab = "", axes = FALSE, ylim = c(rowCount, 0), xlim = c(1, colCount))
title("R colors -- Sorted by Hue, Saturation, Value")
for (j in 0:(rowCount - 1)) {
for (i in 1:colCount) {
k <- j * colCount + i
if (k <= length(colors())) {
rect(i - 0.5, j - 0.5, i + 0.5, j + 0.5, border = "black", col = colors()[HueOrder[k]])
text(i, j, paste(HueOrder[k]), cex = 0.7, col = TextContrastColor[HueOrder[k]])
}
}
}
# 2. Create 7-page color chart showing rectangle block of color, along with index, color name, and RGB constants in hex and decimal.
GetColorHexAndDecimal <- function(color) {
c <- col2rgb(color)
sprintf("#%02X%02X%02X %3d %3d %3d", c[1], c[2], c[3], c[1], c[2], c[3])
}
par(oldparameters)
oldparameters <- par(mar = c(1, 1, 1, 1))
index <- paste(1:length(colors()))
HexAndDec <- unlist(lapply(colors(), GetColorHexAndDecimal))
PerColumn <- 50
PerPage <- 2 * PerColumn
for (page in 1:(trunc(length(colors()) + (PerPage - 1)) / PerPage)) {
plot(0, type = "n", ylab = "", xlab = "", axes = FALSE, ylim = c(PerColumn, 0), xlim = c(0, 1))
title("R colors")
mtext(paste("page ", page), SOUTH <- 1, adj = 1, line = -1)
base <- PerPage * (page - 1)
# Column 1
remaining <- length(colors()) - base
ColumnSize <- ifelse(remaining < PerColumn, remaining, PerColumn)
rect(0.00, 0:(ColumnSize - 1), 0.49, 1:ColumnSize, border = "black", col = colors()[(base + 1):(base + ColumnSize)])
text(0.045, 0.45 + (0:(ColumnSize - 1)), adj = 1, index[(base + 1):(base + ColumnSize)], cex = 0.6, col = TextContrastColor[(base + 1):(base + ColumnSize)])
text(0.06, 0.45 + (0:(ColumnSize - 1)), adj = 0, colors()[(base + 1):(base + ColumnSize)], cex = 0.6, col = TextContrastColor[(base + 1):(base + ColumnSize)])
save <- par(family = "mono")
text(0.25, 0.45 + (0:(ColumnSize - 1)), adj = 0, HexAndDec[(base + 1):(base + ColumnSize)], cex = 0.6, col = TextContrastColor[(base + 1):(base + ColumnSize)])
par(save)
# Column 2
if (remaining > PerColumn) {
remaining <- remaining - PerColumn
ColumnSize <- ifelse(remaining < PerColumn, remaining, PerColumn)
rect(0.51, 0:(ColumnSize - 1), 1.00, 1:ColumnSize, border = "black", col = colors()[(base + PerColumn + 1):(base + PerColumn + ColumnSize)])
text(0.545, 0.45 + (0:(ColumnSize - 1)), adj = 1, index[(base + PerColumn + 1):(base + PerColumn + ColumnSize)], cex = 0.6, col = TextContrastColor[(base + PerColumn + 1):(base + PerColumn + ColumnSize)])
text(0.56, 0.45 + (0:(ColumnSize - 1)), adj = 0, colors()[(base + PerColumn + 1):(base + PerColumn + ColumnSize)], cex = 0.6, col = TextContrastColor[(base + PerColumn + 1):(base + PerColumn + ColumnSize)])
save <- par(family = "mono")
text(0.75, 0.45 + (0:(ColumnSize - 1)), adj = 0, HexAndDec[(base + PerColumn + 1):(base + PerColumn + ColumnSize)], cex = 0.6, col = TextContrastColor[(base + PerColumn + 1):(base + PerColumn + ColumnSize)])
par(save)
}
}
par(oldparameters)
dev.off()