Many people put only one color on. But do you want the model look? If you don't want the model look, then you can just leave and find something for PLAIN.

Model look #1: Put a light color on. Up to the first or segundo eye line. Then take a dark color. Try and make it a darker version of the light color.
EXAMPLE: Lome Green+forest green

Model look #2: take a neon color (preferably orange) put it on the corner and when you go past the corner, kinda swerve on the top.
Them take a pretty color such as blue or purple, and put it a bit smaller than the laranja one. (Still swerve up) And now choose your last color… and do the same

Try out some other things! Be creative