This is my friend Sheri's cast fiberglass sink. It's been looking pretty scratched and faded lately so I scrubbed it up and put some paste wax on it. It looked much better.
I thought I remembered there being a tinted car wax, but the memory was a little hazy. I asked several people and they hadn't heard of such a thing, then I went into the car parts store and found three kinds! I got Turtle Wax Color Magic Car Polish. It includes a hard waxy crayon for deep scratches and gouges and a cleaning polish in black. The Amazon reviews are excellent.
It does a great job if you don't expect a brand-new sink (or car). I'm pleased and I think as I keep treating it weekly, it'll be even better. Gave the black stovetop a nice shine, too. Protip; wear gloves. I'd like to make this really clear; this is not a true fix. The sink looks better but the wax is on the surface and will soon wash and wear off if not kept up.