Old, yellow Skywave decals

Tape the sheet to a window that has sun on it for most of the day. Leave it there for three or four days - it should take the yellow out.

You're kidding ... that's so ... simple?

It does work on most older decals (they weather with age) - only proviso I would put on it is that if you do it in the winter time, make sure the window doesn't "sweat" - that'll screw the decals up permanently. Also if you leave them in the window too long it will cause any reds to fade.