I'm not totally sure on this, but I do know some, from my own chickens. I have some OEGB Red Pyle cockerels, that are about 5 months old. The were all white when they were babies, and some had a few black spots on them, but mostly completely white. Now there is only one that is totally white still, and the others are various shades of red. They seem to be getting redder as they get older. I don't know if this is a thing with certain breeds only, or if they all do this. I know that the RIRs that my dad has were a lighter red when younger, and are getting darker as they get older as well.
Thanks! that helps. My game birds seem to be changing as they get older and I am not sure how they will look when they mature, which is like another month. I suppose they may not change too much more, but they sure look different than they did when they first feathered out. Also my speckled sussex seem to get more white spots as they get older as well. One cockerel is getting lots of white on his underside.