When you view a painting or photograph you might have noticed that the first place you generally look is into the eyes of the subject. Most artists are aware of the importance of eyes when painting, drawing or taking photographs. That's the place to seat expression, feeling and character. The eyes can also convey information about age, well-being, ethnic heritage and the species of the subject - think snake eyes! With all this potential information, you can see why it's so important to get the eyes right when you're colouring an image.
Start by colouring the iris of the eye. Generally, the darkest part of the eye will be around the highlight, and there will be a bit of a shadow from the top eyelid. When choosing colours for the eyes, keep your theme in mind as well as the other colours in your palette.
Use a black fine line pen to fully colour the pupils. You want them solid and dark.
Now use a bright white pen to add the highlight. It's very important to place the highlight in the same place on both eyes... not a mirror image. If you place the highlight at the top left of the right eye, then you need to place it at the top left of the left eye too. Highlights in different locations will give your character a cross-eyed or cock-eyed look, which can be used to your advantage with some characters.
I've made the highlight much bigger and in a slightly different place from the original stamp. This gives the eyes a glassier look making the character seem more alert.
Play around with different highlight shapes and locations, you'll be amazed at how it can change an image, for example, drawing a broken line along the bottom eye lid will give you tear-filled eyes! Try a square instead of a dot, or a ring around the iris, add a few dots in different sizes... just remember to keep them the same in both eyes. Good luck!