from my experience with metallic lettering ... the color tends to darken over the years. I just replaced some lettering on a rig last week that was entirely in gold that I installed 2 years ago. the new gold is bright, catches the light and is all around marvelous looking in comparison to his old gold which is about 2 shades darker. Had I not had to replace his "leased to" to "operated by" above his company name and of course his city and state ... you wouldn't be able to tell the difference, now you can. when you install metallic vinyl, just inform the customer to go lighter that they want because the metal flake in it can and will oxidize with age (darkening the color). Also, since I do a lot of rigs ... I also tell them that the more they drive through an acid wash the faster it will darken.
It's pretty much the same thing as when someone gets a metallic paint job, slaps lettering on it and a few years later when they remove it you see the letters being A LOT brighter than the rest of the paint. all metal flake oxidizes. clear coats help paint not oxidize nothing really to help vinyl though.
as for the vinyl itself ... when I remove the vinyl it doesn't flake away with my stripe off wheel like cast or melt to a goo like calendared ... it actually almost feels like trying to remove sand
paper when it's around 4-6 years old.
otherwise i've never had any longevity issues with it.