We have yet to discuss the benefit that your child will be learning about the different colors as they color their pages. And the more exposure your child has to coloring, the more they will learn about the typical colors of objects, and they will begin to choose specific colors for specific items, such as red apples, or green leaves. And, as they grow older, you will also begin to see that they will color animals the color that they truly are. This may seem obvious to somebody who has known their colors since they can remember, but you had to learn it at some point and this is a great way for your child to learn theirs.
As already mentioned free coloring pages are a great way to teach your child all sorts of things, numbers for example. There are different ways this can be done, one way would be to simply have the number somewhere on the page, such as the number 10, and then somewhere else on the page would be an object or animal cleverly twisted into that number. There could also simply be 10 of something on the page, you can get your child to color the page in and then count out how many apples for example there are, which will obviously correspond to the number on the page.