n00b question

I am attempting to branch out from cheesy Paint!icons to something a bit more complex (cue me downloading GIMP) and I had a question about color. I checked the memories and didn't find anything, but I have a feeling that it's because I have no idea what what I want to do is really called.

You know how people can take a picture and color someone's hair a different color or something like that? What's that called? ::looks sheepish:: Thanks in advance! ^_^

~t.s.