Oh boy, so much to type. Here's a follow up from the fur tutorial. I have both simple and complex eye examples.
Simple:
1: Start with a circle.
2: Outline and fill the lids of the eye.
3: Add a base color and a pupil. This can be on the same layer, or separate, depending on your preference.
4: Make a new layer and put black over the top of the eye. Lower the opacity to your desired level.
5: Make another new layer and put a sliver of white along the bottom of the eye, or wherever you want the light to hit. Set it to Overlay, and lower the opacity.
6: Add highlights.
Complex:
1: Start off from step 3 for the simple eye.
2: Find a lighter color than the base (not too light) and create texture in the iris. Lay down more where the light hits.
3: Add shadows with a color darker than your base color.
4: Take a color lighter than in step 2 and add detail.
5: With a color darker than the color in step 3, outline and define the shapes of the texture in the iris. You can also add a little detail if desired.
6: Take another darker color, and reinforce your shadows.
7: Find an even darker hue, and add more detail to the shadows.
8: Get a color lighter than the one in step 4, and redefine the detail in the iris.
9: Repeat step 8, with another lighter color.
10: Repeat 9.
11: Take black and soften the edges of the iris by 'pulling' black from the outside of the eye inward.
12: With an almost-white color, highlight the spots you colored in step 9.
13: Now time for highlights. Make a white stroke across the iris and pupil. Blend one side.
14: This step is optional. Many animals have a reflective surface in their pupil. To do this, i make a circle in the middle of the eye using either a purple or yellow/green color. Lower the opacity to where it's barely visible.
15: Ignore this step. Lol.
16: Add more highlights to the pupil, or iris if desired.
17: Take black and airbrush the top part of the reflective purple 'disc' so it blends with the pupil.
18: Make a large white circle on the bottom of the eye, set it to overlay and lower the opacity.
Comment with questions.