How often do children move between class 2 reading groups? My daughter says she's 'only a yellow'

Her school has split the class into reading groups this term. They are called green, blue and yellow, which I assume was meant to keep things neutral. It hasn't. On Friday she announced at dinner that the yellows are the slow readers and the blues get the chapter books. She reads perfectly well at home, mostly picture books she chooses herself, and I'm not very worried about the level. What bothers me is that at six she's already describing herself by it. Has anyone asked their child's teacher how fixed these groups are? I'd like to raise it without sounding as if I'm lobbying for a promotion.