You can get "touch up pens" my last one was a tri colour one from Aldi or Lidl. Different surface I know but I had a deep scratch on our old RAV4 I managed to fill it using coloured wax, it took a few coats but it worked really well, I don't see why good old fashioned bees wax type furniture polish would'nt work, alternativly you could cover it using fablon, I'd try the first option.