I collected vintage guitars for many years and every guitar, given enough time, will fade if not in case. For an inexpensive guitar, a case is used as a way to transport guitars with minimal damage. But for a vintage collector, a case also doubles as a place to keep the guitar away from light. For the high end guitars, light is your absolute enemy.
OMG! There's a ridiculous difference between being in a dim room indoors versus being in the case if we are talking years instead of days.
This is especially evident in rare finishes like the Pelham Blue that Gibson used or the Sonic Blue of Fender. Those are exceptionally prized guitars in a blue hue since most existing instruments have been left out and turned more green.
Even some guitars that haven't been known to change to much with light can be so expensive that they always live in the dark. One great vintage shop had everything out there like L5s, Super 400s, and other high end electrics. One day the dealer showed me a Stromberg he only lets live in case and won't bring it out unless somebody asks for it. The same thing happened in Vegas at a huge vintage store that had everything out but kept the D'Anglelico New Yorker in its case. Like high end sports cars that have a lot of their value tied up in being low mileage (and thus travel in large trailers), the higher end guitars with delicate finishes live in cases.
If I had a translucent baby blue or purple American made PRS, even though an otherwise tough solidbody, it would most definitely live in its case. Knowing what I do about these fragile finishes, and having seen dirt brown and puke green on great instruments, I would no more likely swim with the guitar in the pool that I leave them out in the light.
I have never seen a true Sonic Blue in unmolested (by light) condition and only saw a blue Pelman Blue Melody Maker student guitar, but that student guitar was going for $2,500 since it's unlikely that you'll ever see that blue of a Gibson from that period. Sometimes the faded look doesn't make the guitar look ugly but it's still a shame. To see an original '58-'60 with intact original blood orange burst (some examples in below picture) is like seeing a supermodel in person: