Rhino coating comes in several standard colors and can be tinted to any color you want. It's a bit pricy, and is only installed by their dealers, but it must last forever. I had the bed on my 2003 Sonoma done the week I bought it new, and it looked almost new when I sold
it 10 years later.
The spray can stuff is easy on-easy off, meaning that, while it might look ok as undercoating, it's chances of staying on a fiberglass wall are significantly less.