I was going through my eye making tuts last night and I found this Photoshop Action that a complete lunatic has put together. It makes an eye texture that has about 10 layers and is infinitely variable. That is you can change all the layers’ colours and blending modes ad nauseam.
It is really fun to watch and although it was designed for 5.5 it works in 6
One note though don’t follow the initial instruction for the size of the circle just make it say about 20 centimetres diameter on a white background because when it is finished you need to change the polar co ordinates. So after the eye is flattened and saved as a copy without any alphas, you will need to enlarge the canvas to make the iris a circle that is about 12% diameter of the whole canvas. Then apply the polar co ordinate filter Filter>Distort>PolarCo-ordinates>Polar to rectangular. Then reduce the size of the whole image to be what you want.
The left eye was the standard unmodified texture and the one on the right just had a couple of blending modes changed.
Tip: to enlarge canvas in photoshop easily, get the crop tool and crop the entire image then drag the hanldles out on each side. You can keep the iris centred by keeping the crosshair centred in the pupil.