[R] Colours for 3-way probabilities

[Barry Rowlingson]

Are there any R functions for creating palettes for three-way data?
For example, election maps for three parties where pure red, blue, and
green show 100% for the Red, Blue, and Green parties respectively,
magenta shows a 50-50 Red-Blue split with 0 for the Greens, cyan a
50-50 Blue/Green split with no Red votes and so on, with grey, black
or white at a 1/3,1/3,1/3 split vote.

I've spent a couple of half hours knocking out a function to do
various versions of that, including using Red/Yellow/Blue for the
primaries with Orange/Green/Purple for the 50/50s. I'm wondering if

 a) There's existing functionality in one of the packages on CRAN
(I've had a look and googled)

 b) Anyone can point me to information about colour perception of this
kind of three-way colour scheme.

Thanks muchly.

Barry