Convert colors to imitate greyscale printing

Question

When reading this question, I started to think whether it would be possible to convert colors to imitate an average greyscale printer (assuming that your screen is calibrated)? Finding an approvable approximation would save paper.

For example, how to convert these colors to see whether the light and dark blues and reds can be differentiated on paper?

temp <- rgb2hsv(239, 138, 98, maxColorValue=255)
Rl <- hsv(h = temp[1,], s = 0.5, v = 1)
Rd <- hsv(h = temp[1,], s = 0.5, v = 0.4)
temp <- rgb2hsv(103, 169, 207, maxColorValue=255)
Cl <- hsv(h = temp[1,], s = temp[2,], v = 1)
Cd <- hsv(h = temp[1,], s = temp[2,], v = 0.4)

plot(1:4, type = "p", col = c(Rl, Rd, Cl, Cd), pch = 19, cex = 8, xlim = c(0,5), ylim = c(0,5))

Answer 1

Use an HCL palette with chroma set to zero to create greyscale values that are indistinguishable to the human eye.

library(colorspace)
n <- 10
cols <- rainbow_hcl(n)
plot(seq_len(n), cex = 5, pch = 20, col = cols)

greys <- rainbow_hcl(n, c = 0)
plot(seq_len(n), cex = 5, pch = 20, col = greys)

If you want to generate the greys from your original colours, use the scales package.

library(scales)
greys2 <- col2hcl(cols, c = 0)
plot(seq_len(n), cex = 5, pch = 20, col = greys2)