In ROTS there is a subtle shift in Anakin's appearance once he turns to the dark side, the most obvious being the yellow eyes during his attack on the Separatists on Mustafar. It's also occurs in an episode of the Clone Wars where Ashoka temporarily falls to the dark side while on Mortis and her appearance changes as well.

Also in Attack of the Clones, Palpatine is starting to look a little extra wrinkly which immediately reverts back in Revenge of the Sith. I read this as perhaps they were going to go with the subtle change over time but then decided instead to have the change occur all at once. Or perhaps he looked in the mirror one day and decided that he was starting to look a bit too evil and then concentrated really hard to repress the outward signs. Note that when Padme shows up on Mustafar Anakin's dark appearance represses as well.