I go to COLOR ---- COLORIZE --- and play with the levels to change the colors.
That's mainly cuz I am so lazy and I like to do things as fast as possible.
You can also go to LAYER ---- TRANSPARENCY ---- ALPHA TO SELECTION and then us the gradiant tool to make it fade in/out of the color you wanna use.
I can post the actual tutorial for chaging colors the longer way if you want.