Documented research indicate that on 5 Aug 2006 18:41:43 -0700,

Couldn't help but answer this, lol.

The lines become whatever colour the primer is, because you scrape through
the top coat. On most cars with factory paint jobs, you have a thick layer
of primer, and only a very thin layer of top coat. It's very easy to
scratch the top coat, but a lot harder, atleast with a key, to cut deep
enough to go through the primer.

But to answer your question: on our white car, the primer is grey. All the
scratches we have that haven't gone all the way through, or aren't filled
with paint from the morons that hit us, are grey. The rest are either steel
or rust... lol