I read a paper when they did Google in black-dominant rather than white-dominant pages. It turned out black uses more power than white, disproving the initial hypothesis.

I read not too long ago on XDA-devs, that red uses the least.

Perhaps the colour that uses the least is dependant on the screens (or screen types I should say) that you're testing.
Or perhaps red uses even less than black and Google didn't experiment this with the same study.

edit: found it it's http://www.blackle.com/