I was recently asked how I get the details in the eyes when I paint them and this was my response. I thought I would share with anyone who might be interested. : )
It is so hard to try and explain how to get details in the eyes... especially small ones. I don't want you to get caught up in that when the shading is actually more important. Try to look at the iris as shapes and colors rather than an eye. It is hard to separate, I know... it's an eye. But beyond that it is just color and shadow and light.
If I am painting a blue eye and I don't want any other color than blue (and I believe that is hard to make it real and not flat) I would start with the blue I planned on using...
Start with the basic iris shape/circle in black. Then clean that up with white until your black outline is very very thin. On the inside of that black line add your blue... I would use a strong pigmented darker blue and the line inside the line is barely there... practically on top of the black line. Then paint the entire eye in with a watered down wash of that blue. Find your pupil. Now you have the eye. It is plan and flat but there. I know this is a tiny space and I use reading glasses to see it better. Try that.
Now I look at a picture of an eye that is blue... a reference. I look for the light and place it in. First the light shine in the pupil and then I dot it in here and there depending on where the light is at in the picture. After that, if my white is too light, I will wash another color of blue over it and go back again after that and add more white. Look for the shine in the eye on pictures. Look where it is reflecting. It only takes a tiny dot. I generally don't love mixing white and blue together. I prefer layering washes of blue over white.
At the top of the eye under the lash line and around the pupil I mix a little black with my blue... just enough to darken it... and I paint it around that pupil and up under the eyelash line to shade it. It is really a lot of light and dark... don't be too literal. Paint creates an illusion... especially with something so tiny.