It takes me about four layers. I have the base color layer, then I clip the next layer to it where I add the dark colors. I make another layer on add (or luminosity) blending and clip it to the first one and use the same dark color I used on the second one to make the lighter color. I blur the edges of the colors to make them look smooth. Then I make a fourth layer to add the white shines and whatnot.



It's kind of hard to explain in a comment... It'd be easier to explain in a tutorial or something. But yeah that's my basic process for eyes.