And a quick explanation from Plaster himself-
"Here's a rundown of this image(use the dropper for those that don't know what to do with this/are bad at compooter)
Everypony but fluttershy and bon-bon have at least 4 colors in their eyes. those are marked.
and then the main color of the eye is usually somewhere in the cutiemark, the only exception to this that i have seen is luna/celestia and rainbow dash. Also i must add that the colts and other males only have 2 colors in their eyes, and to blend lightly"