Soco TC - black color code?
Its unpainted black plastic, so there is no paint code, of course coloured plastics also has a type of colour code, but black plastic is often just black..
Try polish with car wax, this greatly depends on how significantly the tank is scratched. Superficial scratches can be dealt with in the same way as scratches on painted metal body panels on cars -polish. If that doesn't work - polish harder.
If thats not working and is it a deep scratch, fill the affected area with a plastic filler. Sand back the filler to right surface and prime it. Then paint.
You will likely be able to find a solid blank (not metallic) spray paint touch-up-paint at your local auto or hobby store..
Don't expect to get it invisible, better to buy a new tank cover...