I know Shoreline is a Sunshine pony, but the only part that seems to have changed color is her white mane to an almost blonde color, but her tail is fine. It must just be age because washing doesn't seem to brighten her hair at all.
Overtime the UV sensitive hair is very easily damaged - hence why you see a lot of "wild" haired Sunshine ponies...its probably just that the chemical in the hair has stopped reacting to the same degree as when she was first made...maybe through over exposure to light or reaction to another chemical