Chansen, unless you are a pro and have mastered all the prep of re-painting a FLEXIBLE component (like the seat covers you mention), it is much better to go ahead and buy repro covers from quality folks like NPD, Classic, etc. I tried for years, and got good at patching and fixxing, but for a flexible piece like the seat covers, its pretty tough to get pleased with the results.

For Hard vinyl, plastic, metal, etc, you can most certainly work those pieces to near perfection with the proper prep, paint adhesion spray, then true color spray. All original colors for these projects are available from several sources.