I have actually taken three classes from three different teachers to understand the eyes anatomy thoroughly. The colourful eyes page is filled up after taking all three classes. 

I have taken the classes in following order;

1- Art Basics - How to Draw Eyes - Bring Your Characters and Drawings to Life! by Jordan Hill.

2 - Drawing Expressive Eyes for Beginners by Margarita Bourkova.

3 - Learn How to Draw: Features of the Face by Melissa De Nobrega.