I'm quite surprised that the people so far think this sort of colour change it normal. It's not. About 6 months ago one of my Panaque nigrolineatus (L190) did this and reverted to back to normal, and it still is outwardly healthy. It's not thought to be harmful to the fish and is unknown what causes it except maybe stress, but here is some discussion about on on pleco colour morphing and a German article about other morphing species
This is mine.