Hi and welcome! I am fairly new too, but I can cut pretty well with my rotary cutter. For me, the trick is pressing down with the ruler. You have to have a firm hold on it when you cut. There are also some little rubber things you can get that attach to your rulers to keep them from sliding around on your fabric. I got mine at JoAnn's, but other places probably have them too.
As for the purple ... well, I guess if that's what she wants, I would probably do it. I once let my teenager paint her bedroom a purplish-gray and red! It was hideous, but it was her room and her choice. Later, after she'd moved out and come back from college, she asked why we let her do that, and we told her that it was her room and we wanted to give her a say in the decorating of it. (I am happy to report that her taste has improved!) LOL