Tada! I use the ReColor App by Sumoing. No, I am not an affiliate, so if you decide to use this app, I don’t get a darn thing. Except maybe a new follower … If you’re so inclined, my ID is White Rabbit w/ Grey Hare.
Anywayz, the kind of stuff I do in the app is pasted below.
As you can see, they have diverse template options, and lots of different finishes, filters, and even line colors. I’ve tried quite a few different coloring apps, and while this is the most expensive ($80/year last I checked!), it also seems to be the most reliably helpful to de-stress and create pretty pictures.