The game works on a very strange system of requiring paint chips for the paint shop. You get them either by logging in daily to the PSN servers in-game (anywhere from 5 to 10, random), or by acquiring cars. If you no longer have the original (Silver Stone Metallic for most Miatas), you'll have to either buy a new one to get the chip, hope for it to be a random prize, or paint it a suitable replacement.
Hope that helps!