There's a good article on molting at
NCS - Cockatiel Molting It says that the first molt is around 6 months, although my experience is that weather plays a part and it can be earlier or later than that.
Diet plays an important role in feather color, but you don't see the effect of a change in diet until the bird molts. If her cheek patches are getting brighter, it might mean that she's been eating more beta carotene or other nutrients!