Basically everyone who learns to create first learns by copying–tracing and then freeform copying others’ drawings, making recordings of yourself singing/playing along with recorded music and next performing someone else’s compositions, etc.; we just have more tools for both creating and for making those creations public by default now.
In the case of coloring books subject to copyright, I’m guessing that the colored-in physical artifact can be displayed, sold, etc., but not copies. That’s a bit sad.
Contributing to the popularity of e-coloring books are their de-stressing, relaxation effects and the fact that users’ work-products can be saved and shared.
Presumably in a very limited fashion, though I’d be happy to be wrong.
Also coloring books are apparently used for all levels of education, even graduate level. I searched for OER coloring book and found nothing, but I may not have searched hard enough.
There are some usable images in Wikimedia Commons coloring books category and probably lots of line art or images easily convertible into line art that could be used, and also much on openclipart.org.
Of course for the purpose of building free myths, coloring books using characters from free movies such as Big Buck Bunny or Sita Sings the Blues would be most excellent.