I'm assuming you just mean the eggs are different shades of brown right? Their egg colors wouldn't change totally from brown to white.My cochin isn't quite old enough to lay yet so I don't know what color her eggs will be but I know that there can be a variation in a chicken's egg color from day to day. I see it all the time with my Australorps and Brahmas. Typically the Brahma eggs are darker brown than the Australorps but if I put 12 Brahma eggs in a carton there may be only 3 or 4 that are the same color (of brown). I think that is probably true of most breeds that lay colored eggs. Just my opinion.
I think it most have something to do with bloodlines. I wonder if Luckypickens could offer some input......