Glowing orbs in with a slight veiny appearance to the outer area of the eyeball, all bathed in the color of choice, with a cat or reptile or whatever you want it to be, slit-shaped pupil. Shadow and highlight on the eyeball itself. Yes, I did say glow-- these are illuminated therefor they penetrate all form of lighting or lack there of.
(Illuminated effect works on default head and may or may not work with other heads).