Three phase colot scale

[Erik Buitenhuis <ebuit@xxxxxxxxxxxxxxx>]

Hi all,

I would like to plot three variables, each with their own single color
(e.g. red, blue and yellow) and be able to display mixtures of these
three colours if more than one variable is greater than zero, if
possible even to the extent of having black when all variables are zero
and white when all variables are one. Any ideas
on how to do this would be appreciated.

Thanks in advance,
Erik Buitenhuis.