I'm no expert on Betta eye color, nor am I an expert in genetics but I assume there is some connection between the colour of the Betta and their eye colour. The most common eye colours I have seen in Betta's is dark coloured - either black or blue. I've also seen Betta's which are heterochromic (either each eye is a different colour or one eye is multiple colours), and I think that type of colouring is more rare.