According to a couple of guys here, the ultraviolet light doesn't work like the sunlight. What is more the UV light may damage the decals or just causes the decal getting dried. Usually it results in cracking of the markings. I have experimented with the 15 years old KP declas and left them behind the window glass for one day initailly. The day was very very sunny and the yellowish tone disappeared in a half. So I decide to leave it longer and three days later got them without the yellowish hue.
The long way to restore them is difficult, but is the only way to really make them right. Scan them at high resolution, load them into a photo editing program, mask the images and drop out the yellow background. If some images aren’t clear enough you may have to redraw them. Print them on new inkjet clear or white background decal paper. White background paper is necessary for any images that have white in them. Inkjet ink is water soluble so the must be coated with a decal sealant like Microsol decal coating.
If I’m developing a habit to scan all new decals. You never know when you screw something up and having a backup can be wonderful.
When printing an all-white images on white background paper you’ll need to outline the image with a black hairline to provide a guide to cut the decal out of the sheet. It’s why commercial decals are printed on a blue background so you can see the images.
Like I said, this is a long and skilled process requiring some graphic software expertise, but it really works. I’ve been the dedicated decal customized for our model club.
Builder, some sage advice about making backups. Never thought about that and in the past it would have been a savior.
As to graphic software can you recommend a program for converting raster to vector images and one that will not take a lifetime to learn. I have some 1/72 decals I want to convert and scale up to 1/48 but not having real good luck there. I'm using Corel Draw currently.