make colors consistent for categoricals with 3 values

[terraswat]

Many maps have a lot of categorical attributes with one of three values. When switching between attributes on the map, it is disorienting for the colors to keep changing for say, the high value of each of those attributes.

For instance many attributes could have the values of high, medium and low. Always using the same color for each value would be much better.

from Chris Wong:
My hacked-together fix for the color-mapping issue is at /data/home/chrisw/projects/luad_mrna_top1000byvar/Makefile. check out the scores_colormaps.tab target.