following what NextDesign is saying

create a sphere

create a blinn
click the square next to color and assign a ramp to it

now for the last one use white, the 3rd one is black (the black line around the iris the second one is white and the first one is black... now move them to match where your color will be

not take the second spot
and click the square next to color... click on fractal... and now play with the fractal settings =) color gain to chose your iris color

its something I found while I was searching for a cartoon eye and im pretty sure a lot of people already know it but if it can helps im happy =)