The natural sunlight is the cheapiest way for removing of the yellowish decal tone. You may try to the UV lamp but it may be not enough to get the decent final effect. Actually all depends on what got the yellow tinge. It might be the clear decal film ( background ) , the glue or the base for the colours. So the sunlight is better because its spectrum is a broad light one.
PS. The exposure to sun may last a couple of days or longer depending of the intensity of the Sun. But later you should either applly the clear coat all over the decal sheet or use a decal softener in order to prevent cracking.
