The sun bleaches and destroys the pigments that reflect the color you see. I believe it is the UV wavelength that breaks up the color and changes it's reflective properties. It is destruction on the molecular level and I know of no existing technology, technique, or practice that can reverse the molecular change.
The candy colors were made by tinting clear with small amounts of color pigment. Then repeating these coats with successive layers of tinted clear until there is enough shading and darkening to yield the desired color. There may or may not be a true clearcoat as a final layer.
Some metalflakes are done the same way; with tinted clear (candy) layers over the metalflake base.
The fading is the result of the pigment choice that was used when the paint was applied and sunlight. There are modern pigments today that resist the sun's damaging effects. But, there are no techniques available today the allow you to replace only the existing damaged pigments with the new ones without repainting the subject.
You can over-paint, or strip and paint.
Maybe you can experiment with dyes. But, I don't see how you could get the "depth" appearance of the original finish, as the original finish is not permeable, except for light.
Hmm, maybe a laser technology could do it. How much money do you have for development of the restorative technique? A mil or two might do it. Interested?
Cheers,
