Old, yellow Skywave decals

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.

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

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.