I started digital coloring (painting actually) late last year when I got an iPad Pro and Pencil. There are a number of nice Apps to use. After trying several, I settled on “Pigment.” It has lots of blank outlines that interest me. I especially like being able to paint digitally in oil, pastel, and watercolor, in order to do shading and make highlights standout. In sum, it’s fun and relaxing. It’s a long way from “kid coloring” but is also only as intricate as you might want. It’s creative. That’s what I enjoy.