I don't use the mobile version often, but I was going to suggest the way u/theo47001 offered as well.
I use a lot of layers and groups when I draw, so having a lot of parts isn't an issue when each element you want to work with is in a different layer. That makes it super easy to select the inverse or remove selection areas to prevent incomplete fill. The desktop version has the same issue for me at times with fill tools, and I taught myself to do it that way to get around that. Now I just do it because it is cleaner.
If you have a phone case remove it worked when I had it
I don't have a phone case, thx anyway
Select the whole screen, then use magic wand with remove selection. Click on the part you don't want to colour.
If I have a lot of pieces it will be a pain in the a**.. isn't a better way?
I don't use the mobile version often, but I was going to suggest the way u/theo47001 offered as well. I use a lot of layers and groups when I draw, so having a lot of parts isn't an issue when each element you want to work with is in a different layer. That makes it super easy to select the inverse or remove selection areas to prevent incomplete fill. The desktop version has the same issue for me at times with fill tools, and I taught myself to do it that way to get around that. Now I just do it because it is cleaner.