My 15 year old wants to paint one wall of her room dark green, herself

She has watched what I suspect are thirty videos, measured the wall, picked a shade with a name like 'forest night', and offered to pay for half the paint from her birthday money. The plan includes masking tape, two coats and moving her own furniture. My instinct is to say yes. My hesitation is about the ceiling, the floor, the fact that paint takes forever to dry in this weather, and whether a dark wall in a small room is something she'll still like in six months. She says that's what repainting is for. I'm not really asking whether to let her; I think I will. I'm asking what to prepare for, from anyone whose teenager has done their own room. What went wrong, and what turned out better than you expected?