FIRST GREAT QUESTION!
Second! I was wondering it too! LOL! My mother in law's two brown's dapple- the mare more than the gelding.... and my copper color bay never does. They area all fed the same way. I sometimes THINK I see dapples in certain light on him in the summer, but then I am not so sure when he moves, lol. Not sure if it's harder to see on some colors?
What exactly makes the color slightly different? Is it the angle of the hair, or is the pigment actually a slightly different shade, or the oil in it's skin?
Angels are people who can see and feel the pain in all creatures... -myself