My sister made a Sunbonnet Sue quilt using crayons, back around 1980, and it has been washed several times, and is still holding the color very well. She colored on paper, then ironed the crayon onto the fabric. She was able to transfer 2 blocks from each colored page.
I did take a class where we colored directly onto the fabric. The teacher had us color light layers, then iron with a piece of paper on top, between each layer. I really like how I was able to get some nice detail with this method.