For instance, blue is directly across orange. This would mean that people with blue eyes would look gorgeous in orange eyeshadows or anything surrounding that range (champagne, gold, bronze, etc). For brown eyes, blue happens to be the color straight across- so blue eyeshadows, teal, and purple will all make brown eyes really pop. For hazel/green eyes, check out the olive green color on the wheel- purple is across from it so colors like purples, pinks and burgundies would really make hazel/green eyes pop.
Color theory is really simple. If you want to enhance a certain color, just look right across the color wheel and you have your answer!
-Patti, contributing elfette