The New York Academy of Medicine apparently hosts the Colour our Collections event for several years now. I only just found out about it yesterday from a book site that I follow. Museums and libraries from all over the world participate, putting up images from their collections. Though the event is over (running only between 04 February to 08 February); meaning for you to share your coloured image with a hashtag, you can still access all of the colouring books from over one hundred different institutions.
I looked through every single entry that is posted. There's a lot of cool stuff, some repetitions (though few), a lot of local only things which are fascinating, and a lot of weird stuff as well.
Though I saved several books and screen captured single images that I would like to colour at some point, I did not save everything, but I found everything interesting. I already knew humans were weird, because I've seen art from the Medieval ages, and some of the things in these collections do not disappoint.
Like the image above that I like to call, "Wife Moon Face tells Husband Sun Face, 'Look, it's a chicken...'" because honestly there's not anything unweird about that above piece. It's a long title, but it makes me laugh. There's also the skinned humans in poses of "Oh, God why!?!", the skeletons just posing by things like ya do, there's even one child-like skeleton, skipping down the road with an adult leg bone. Lots of horrible beasts that people once imagined roamed the lands and seas. Lots of crazy, and also at the same time hilarious drawings from long, long ago.
But there are lots of things for anyone to enjoy looking through or colouring. Things for children or the young at heart, botanical illustrations, early 20th century yearbook drawings, scientific renderings of fauna, historical, old political cartoons, old advertisements and newspaper/magazine covers. There's also photographs turned negative, which I don't much care for, but one of you might.
I suggest giving it a look through. There's good stuff here for your children, a classroom, a library group, yourself and your friends.