[R-sig-Geo] ramping four color in xy space

[Holger Kreft]

Hi,

I have run a metaMDS(vegan) on ecological community data and have a nice 
plot in xy space.
What I want to do next is to assign a unique color to each point in the 
plot and then look at how these communities map out in geographic space.
So, each corner of the plot has a color (let's say green, blue, red and 
yellow). I guess I have to ramp the colors for the space inbetween.
The right plot here illustrate how the palette should look like: 
http://www.flickr.com/photos/erniebooth/183647844/

Does anyone have experience in ramping four colours in 2-D? I looked at 
a lot of packages, but they are all not doing what I want.

Any hint / help is greatly appreciated!

Thanks,

Holger