I am really questioning the use of any 4 custom colors.
A gameboy did have 4 colors, yes, but actually only 4 "grayscale" colors. (White, light gray, dark gray and black).
The only thing that made it "greenish" was the screen that tinted it.
Don't know why you all are just picking any 4 colors but I guess it's a matter of how authentic Gameboy you want it to be
EDIT: Appearently the rules doesn't apply to imitating the gameboy hardware. You can use any 4 colors at any time (even switching within the game). As long as there are only 4 colors on screen at the same time. :-/
FreneticGamez: Not if you aiming for a Gameboy game, no.