I think if you SITD around the wheel wedges in a dark color, that would really give them a lot of definition & could improve the look of the piecing! I agree that a design in the pink centers & in the background fabric would look very nice.
As far as a white on white backing, I guess that depends how it will be used. The downside of white is that is gets duller with time & eventually just looks dirty even straight out of the wash. On top of that, white cotton when it absorbs sweat is very unattractive & difficult to clean. There are so many options to choose from in the wheels besides pink & peach that I think you'd have a hard time going wrong. Personally, I'd go with a red or blue, but if all that pink indicates this is for a little girl: (1) there is no such thing as "too much pink", and (2) purple makes a nice alternative to pink to keep it girly but still keep it from being to monochromatic.