Probably not good to paint a plastic tank because, like painting a ladder, it can hide potential imperfections and failure points, (a gut feeling). And not a colour other than red (or yellow for diesel fuel), I would guess again. I like that idea, though, because it will protect the plastic (of a fairly new tank, say) from UV damage, where other ways are not practical. But any scratches would create a vulnerable "achilles heel" that might go unnoticed. Of course, painting a tank would prevent you from determining, by visual inspection, the amount of fuel in the tank.
I would also point out that, while this may or may not be the case here, some polymers can fade considerably from sun exposure, and not lose much strength from it. Ends up being solely cosmetic, as only the dye added to the plastic has faded, not the structure of the parent resin itself. Test the tank with a blunt pointy object (like a dull marlinspike) in the faded area to find out for sure. Push slowly.
It's a very demanding task for a paint to stay stuck to a surface that is constantly expanding and contracting from temperature and internal pressure, unless the paint does so at the same rate. The only paint I've had any luck with over plastic is a product called Krylon (spray can), which might be available in the USA(?) Clean the surface well to improve your odds of success. I've seen the paints you mention that are made for painting over resin lawn furniture, and they might work well, but I have no personal experience with any other than Krylon. Anyone else?
-Brian.
