When Ash's "ride" (the sled being pulled by his Pokemon) is trashed as he's trying to get to Ice Island, Team Rocket picks him up. At this point, Jessie's hair is its regular, bright red color. As they go through the tunnel, the shading darkens everyones colors, including skin tones, hair, clothes, etc.. But when they reach the room with the treasure, and everyone else's colors have returned to normal, Jessie's hair is still a violet, almost purple color. This only switches back after they get onto Lugia.