How? Age old question for me....btw I use paint and Gimp 2............

MERRY CHRISTMAS!!!!! (Can't sleep :P)

use GIMP, since you can create alpha in there, im sure you can view the RED, GREEN, and BLUE channels, right? go to RED.. red is shine and stuff (blue also helps there, but blue is intended to be color change) anyways, when directly on a channel, the shades of white represent how much red is applied on thos areas.. use a black brush or what ever untill your red is ajusted to how you want.. move onto green, preaty much make this one all black; green is glow, just leave the few little glow spots if you have any. Blue, Color change and shine helpers.

Anyways, you just use the channels tab, and ajust each channel individualy to the settings you like. (I myself prefer being lazy and using the bright/contrast feature in photoshop to enphasize areas, then full green with black and then use history brush on the like 2 lights)