Basically, the variance in melanin is what causes different eye colors. So, if you have a gene that produces melanin, and then turn it's expression down, then y

Basically, the variance in melanin is what causes different eye colors. So, if you have a gene that produces melanin, and then turn it's expression down, then you might get bluer eyes. The complicated part is that a lot of genes actually control for eye color - that is where the complications come. Each gene interacts with every other gene in a unique way.