[R] color scale mapped to B/W

[Achim Zeileis]

On Fri, 6 Jun 2008, Michael Friendly wrote:

> In an R graphic, I'm using
>
> cond.col <- c("green", "yellow", "red")
> to represent a quantitative variable, where green means 'OK', yellow 
> represents 'warning'
> and red represents 'danger'. Using these particular color names, in B/W, red 
> is darkest
> and yellow is lightest.  I'd like to find color designations to replace 
> yellow and green so
> that when printed in B/W, the yellowish color appears darker than the 
> greenish one.
>
> Is there some tool/code I can use to find these? i.e., something to display a 
> grid
> of color swatches with color codes/names I can look at in color and B/W to 
> decide?

You could look at colors in HCL (i.e., polar LUV). For example, you could 
choose a dark red HCL = (0, 90, 40) and a light green (120, 70, 90) and a 
yellow somewhere in between.

To emulate what happens when you print that out, you just set the chroma 
to zero.

There are some functions helpful for that in "vcd", you could do

   ## load package
   library("vcd")

   ## select colors from dark red to light green
   c1 <- heat_hcl(3, h = c(0, 120), c = c(90, 70), l = c(40, 90), power=1.7)
   c2 <- heat_hcl(3, h = c(0, 120), c = 0,         l = c(40, 90), power=1.7)

   ## visualize in color and grayscale emulation
   plot(-1, -1, xlim = c(0, 1), ylim = c(0, 2), axes = FALSE)
   rect(0:2/3, 0, 1:3/3, 1, col = c1, border = "transparent")
   rect(0:2/3, 1, 1:3/3, 2, col = c2, border = "transparent")

The ideas underlying this color choice are described in this report we've 
written together with Kurt and Paul:
   http://epub.wu-wien.ac.at/dyn/openURL?id=oai:epub.wu-wien.ac.at:epub-wu-01_c87

hth,
Z

> thanks,
>
>
> -- 
> Michael Friendly     Email: friendly AT yorku DOT ca Professor, Psychology 
> Dept.
> York University      Voice: 416 736-5115 x66249 Fax: 416 736-5814
> 4700 Keele Street    http://www.math.yorku.ca/SCS/friendly.html
> Toronto, ONT  M3J 1P3 CANADA
>
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