are you using the freezer paper method? because two colors can get complicated. but i do like the idea of making the black the outlines. looks wicked cool. i might steal that idea.
if youre thinking of doing green outlines on the shirt. more simple, the best way to remember is that everything thats black will then be green, and everything thats white would be black
kinda like this:http://www.craftster.org/pictures/data/500/medium/234233_15Nov10_dark_mark_ii.png