How can i get my parents to cooperate?

Well you could explain that you want a fresh start, and that the walls will not only help you achieve this, but they will also make you feel better about who you are. And as for the fact that they just painted it two years prior to this epiphany of neon green, you could offer to do a lot of the work yourself. Now, I know that that doesn't sound like a ton of fun. But reinventing yourself can be difficult. And what is manual labor if not difficult? But really, it might not sound like fun, or it might, I don't know. But if this is the condition that will make them say yes, then you have to think about whether doing most of the work yourself is worth reinventing yourself.