Ok, I don't pretend to offer the perfect solution, but I hope to give some interesting ideas.

You need to start with a good picture of an eye. There is much complexity of colors in the iris, so even if you want to do it by yourself in Photoshop, I suggest still to get a photograph and study it. Another thing I would do is to avoid making the white of the eye just white. Add some hint of yellow and pink and of course, if you have some time to spare, you can have fun creating the fine network of veins in the eye. Here is a small Photoshop example: I did it in 2 minutes, so I am sure you can do WAY better than that.