Hmm, I can only think of Heck Yeah! Pony Scans! on Tumblr, who should have at least one or reblogged a few scans of one. Then, there's probably a couple more blogs on there that may have scans of G1 coloring books. I actually don't think there's very many online at all.
I would love to see more. I imagine between them being a paper good, over 40 years old and difficult to find in a condition worth scanning, it may be hard to find them at all. I wonder if there's anyone that can swoop in to help out more?