http://forum.cockos.com/showthread.php?t=55751
there is somewhere a pdf of that for download made by White Tie
you edit it in something Photoshop, [I do] or in GIMP if you use that...
then save it as a .png file with a new name...
then you can have and use you own color map and\or change it on the fly for any other color maps you have on hand...
there are more than a few user made color maps on the stash.
Now I'm assuming you do mean the 'notes' in the midi editor, not the piano roll keyboard, right? those would be a different story but can also be done.