Sun and UV light sources will fade plastics (and paint) over time. With the demise of fluorescent lighting (a source of UV light indoors), we see far less of this.
I was a wedding photographer for several years, and the wedding photos hung up in hallways hardly faded at all.
But those that were exposed to (even indirect) sunlight or fluorescent light would fade horribly. Album pages hardly faded at all.
With the exception of black molded plastic, all other colors require a more expensive colorant to be UV resistant. Apparently all black colorants are UV resistant.
So an injection molder, trying to make a little more money, might skimp on the colorants and use non-UV resistant versions. There is no way for the customer to check other than to require certification and colorant receipts with each shipment.
We’ve all seen store signs (red, especially) where the color faded badly. The sign maker used cheap plastic for those signs.
Also, the self-adhesive sign film is available as “plain” and “UV resistant”. The same economics drive the sales of the cheaper versions.