[R] fixed color scale for a series of contour plots

[Marion Dumas]

Hello,
I am having trouble figuring out how to create a color scale that will  
be consistent across multiple contour plots. What I would like is to  
plot a series of contour or image plots for different subsets of a  
data set but make sure that the color key stays fixed so that the  
plots are easily comparable. That is I need a map from the range of  
values of the entire data set to the color scale and keep then use  
this color scale for all plots + be able to display the color bar as a  
legend next to the plot. This sounds trivial, but I can't find useful  
hints in the documentation of the color arguments of the different  
plots.
Thanks so much for any help
Best
Marion