I'd never heard of this version of picross before. I found this really fun and really addicting. I played this for like an hour, loved it.
With that out of the way, I would give a pinky for some kind of "erase" functionality. Especially on the levels where it's asking you to redo the puzzle with your art still on there, it's tough trying to work out the solution with the old stuff right there. I would generally try to just paint it all one color as a kind of "reset" but I didn't love that.
I also had trouble differentiating colors sometimes. I don't think you're helped by your level 1 being yellow, grey, and different grey. That confused me at first and made me think I didn't know the rules when I was first giving it a try. In the later levels, trying to tell the difference between "sort of salmon" and "sort of red" is tough as well. And I actually have good color vision! I feel bad for color blind people, ha.
I wish I could click through Bob Ross' text boxes as well. And the "next level arrow" maybe should be signposted a touch more. Could it start "grow and shrinking" when it's ready for the next level? Does it already? I had some issues with that at first.
Amazing game, great job.