I believe the base layer is like primer. Just fill in all colored parts on the car with one color (grey or white). Make those sections one color. You can later change the layer blending options to change how the highlite / neutral / dark layers appear.
At the end I think you can either make a new layer as color for the car, or just go the the base layer and change it's color as a whole.
Current Cars:
2019 BMW X3 M40i Stage 2, 12.44 at 110mph
1972 Chevy Super Cheyenne C10 Pickup 402 big block, 700R4
2004 GMC 2500HD 8.1L
Past Cars:
1970 Chevy Blazer, 2wd
2003 BMW X5 4.6IS Doushmobile, moneypit
2015 Ford Fiesta ST | Cobb Stage 1, catless downpipe
2008 Corvette Z06 - 11.39 at 123.8mph
2002 Corvette Z06 - 12.10 at 116.5mph
2005 Jeep Wrangler LJ
1993 5L Mustang - 12.59 at 108mph
1989 5L Mustang
1990 Jeep Cherokee
1991 Acura Integra RS 403Honda