When painting doll eyes, it's good to know a little about the anatomy of the human eye.
Sclera ~ the white outer layer of the eyeball.
Iris ~ the flat, coloured, ring-shaped membrane behind the cornea of the eye, with an adjustable circular opening (pupil) in the centre.
Cornea ~ the transparent layer forming the front of the eye.
As you can see by the pictures above and below, every eye is different and each one has a combination of many colours, with each of those colours forming slightly different patterns.
The sclera is not actually a bright white, it also differs with every eye, some show noticeable veins while others appear much clearer.
One could study eyes all day and not see two identical, not even when they are a pair. Try it yourself, look your own eyes when you have some free time, I'm sure you'll find them quite fascinating and it will certainly help you understand how to paint them better and make your dolls appear more lifelike.