I tried a few and deleted them. The only two I have kept so far are Pigment and Lake. I'm not sure but I think they support images from the photo library, but you have to pay extra for that. Pigment is good but I don't want to pay a monthly subscription fee. Would prefer it if they also offered the ability to buy individual drawings for 99 cents.
Pigment is fantastic for coloring and I have imported pages that I have found on the internet and while buggy and prone to crashing on import, it will get them there after a try or two. You can also import from iCloud and Dropbox drives.
You can also import photos to color, but I find that is a big hit and miss as far as being something you would want to color.
Edit: Import is likely a Premium only feature, however.
I tried Pigment, but I prefer Lake. How well does Pigment work with Pencil ?