Un-yellowing Decals

The transparent decal carrier-film of most manufacturers can become yellow in time. The degree of yellowing can range from slight to brown. 
Fix the decals to a south-facing window and sunlight will reverse the process 100%.
In summer in the UK, a week of strong sunlight will usually suffice; a lot more time will be needed in winter.
Use blue-tack for fixing, NOT masking tape. The latter will establish a permanent liking for the window and the decals. If the decal sheet curls up, put it in a clear plastic food bag and tape it to the window but not with masking tape. Having said that, some DIY stores sell, at extra cost, a low-tack masking tape that will still be removable after several days.
DO NOT attempt to carry out this procedure when there is a risk of condensation on the window!
The images below show "before" and "after" for a set of slightly yellowed Frog Wyvern decals. Even the white of the roundels has brightened: