Late summer is a time of transition for birds and a time of tremendous variety for birders. You can see adult birds in breeding plumage (looking worn after a long breeding season), fresh nonbreeding plumage, and any stage in between. And in the same flock, you might find newly fledged birds still in juvenal plumage, and others that fledged earlier in the summer and are already in fresh first-winter plumage.
Considering the dramatic changes in feathers, and the variety of appearances, it’s not surprising that most birders miss the changes in bill color that happen at the same time. A bird’s bill is not a lifeless bony growth, like a horn or an antler. Bones form the foundation, but over the bones is a layer of living tissue, and covering that is a thin layer of hard but translucent keratin. The living cells just below the keratin layer can change color, and because the keratin is translucent, the colors are visible.