Why have many of the smaller goldfish changed colors while the biggest ones are still mud colored?

I have a large pond (about 10' across about 5' deep at the center) in my backyard. I had two larger goldfish, two smaller ones (the only offspring to survive last years' spawning), and three koi. This spring they had a major orgy and about two months ago I discovered about 100 small dark gray (mud-colored) babies...about 50 left when I counted today (the seagulls love my house). About half a dozen have turned mostly gold (small patches of black on their fins still), one turned white, and about 20 have started showing patches of orange, gold, and/or white color. But I'm confused by one in particular. The ones that have already developed color are about 1.5 to 2" long...most that size have at least some color. Most of the really small ones are still gray, but there are about 10 that are about 3" long (and one big boy that's at nearly 5" long) that show no signs of developing color. Why are there so many sizes and why haven't the largest ones changed color yet?