You can use the eye textures in the Textures folder. I find that very useful because the pupil is already done and so are the teeth...or maybe I'm just lazy. XD
It's simple enough; open the eye texture you want in Gimp or Photoshop, make a new transparent layer and make sure it's underneath the eye texture. Then to make you're eye colour, colour on the new layer you made - it should show through to the eye texture layer. Then, when you're done, just merge the layers together and save it.
I hope my explanation was okay. If not, I can try and go through it again in my detail; that was just brief.
