iPad Brightness Issues

I use my iPad for drawing. Whenever I’m rendering something dark in color (dark skin, dark clothes, etc.) the iPad, despite being on full brightness, will “tick” down a notch and I won’t be able to see what I’m doing. I have to go onto a lightly colored screen to get it to brighten again (safari, a game, etc. it actually happened while I was typing this) This has been going on for years and is INFURIATING! I don’t get why this doesn't happen on bright white backgrounds, where I assume the iPad might drain battery or something— only dark ones, where it can inconvenience me. How do I get it to stop?