Well might just be my uneducated self, but.....Last summer, end of season, I noted my free standing feeders had bees that were almost black among them. Most were very dark. When filling the feeder, I noticed that the bees that were feeding (just yesterday) were mostly, but not all, very yellow, so yellow my first look was followed by a double take to make sure it wasn't hornets.
As I watched longer, there were very black bees and very yellow bees coming out of the hive, nothing between. I am assuming, not positive, that the black bees are older and the yellow are very young. Mind you this is only one of my hives. The other one, the few times I have seen them venturing out the colors are what I saw last year in that hive. My assumption is as they get older they darken, at least in that hive which is my strongest.