This reminds me of a story about a different film director/designer... In one of his films there is a clanking sound when someone is being shaken. When someone asked him why it is there, he replied, "The critics will think of some reason".
This seems to go the opposite way, though. Honestly, I didn't notice any difference between the Uruks. They were all equally orcs and smudged with white paint to me. I don't remember noticing any grey-haired ones.
Same goes for Gandalf/Theoden/Wormtongue.
I think they put all kinds of details in that are meant to signify something, but these details need more emphasis if they want the audience to catch them.