Enlighten me, please! Why aren't my parents letting me paint my room?

Yes, they are not really giving me a good reason. I keep asking them but, they keep saying no. First, they said I couldn't paint my room because they didn't have the money at the time but, I do and I told them I could buy it. They said no. Second, they said it was because of our type of walls. We live in a double-wide trailer and I guess you could say our walls are "weak." They kind of feel like they are made of plywood or something, but I honestly have no idea. I have a friend that has the exact type of walls as me, hers are painted, and they look great and actually normal. I showed my parents a picture of her walls but, they still said no. I really have no idea why they keep saying no. I severly hope it's not because they are trying to keep me grounded or anything because I'm a good teen, I never get in trouble at school, I rarely ask for stuff, I never go anywhere, I've even shared a room with my sister for 12 years, she's moving out in a week, that's why I'm asking them, it's not like I'm being random, and I've gotten all A's since 4th grade(I'm in 7th now). Any one of you have an idea why?