[R] Selecting complementary colours

[Thomas Lumley]

On Mon, 21 May 2007, John Fox wrote:
>
> In retrospect, I didn't specify the problem clearly: What I want to be able
> to do is to place text on a background of arbitrary (but known RGB) colour
> so that the text is legible. I guess that this is better described as a
> "contrasting" than a "complementary" colour.

Since luminance contrasts are necessary and sufficient for readable text, 
you could use white for dark colors and black for light colors.

Luminance is roughly proportional to  0.2*(R^2.4)+0.6*(G^2.4), suggesting 
something like

lightdark<-function (color)
{
     rgb <- col2rgb(color)/255
     L <- c(0.2, 0.6, 0) %*% rgb
     ifelse(L >= 0.2, "#000060", "#FFFFA0")
}

This uses a pale yellow for dark backgrounds and a dark blue for light 
backgrounds, and it seems to work reasonably well.

 	-thomas