I know many years ago POR-15 was marketed as not being UV color stable. In other words if you left it out in the sun it would hold up, but it would fade out and turn yellow or chalky.
I can vouch for this on a couple quarts of POR15 gray that I had used on a steel project. It did turn yellow after a year out in the sun. Still tough as nails though.
Maybe they have reformulated it now?
1999 Ford Superduty F250 PSD CC SB 6spd 4x4.
B&W goosneck/companion hitch, Airlift 5000 airbags.
2006 Sierra F28 Rear Kitchen 5th Wheel, 31' = 10k pounds.
Nights camped in 09-14, 2010-23