This week I received an email from a customer who had removed a decal using the Wonder Wheel from the side of a car. The Wheel took off the decal fine, though it appeared that a fine film was left on the surface (and it is not glue or adhesive)…..
I have not seen the film layer though from experience, this occurs when an old decal is removed. What can happen (and very rarely) is that over time, usually  5+ years, the paint on application surface (e.g. car door, plane wing, side of truck or train etc) can fade slightly. This maybe only a very slight fading of the painted surface, though as the paint where the decal was, hasn’t faded, then it can like a different colour; or some may say it appears like a layer of film.
For this situation, an option is to polish the area around where the decal was to bring improve the surface shine. This will reduce any difference in colours