A lot of people take colors from their favorite auto brands. For instance, you will see many Porsche enthusiasts use the basic colors from the badge which also happen to be from the German flag: read, yellow, black.
The old Grand Prix colors denoting country of origin had British cars green and French blue while the Italians were red. But instead of all red, Italian inspired combos will feature red white and green again from the national flag.
American factory schemes can sometimes found at dealers like the Mr Goodwrench black, gray and red or Ford blue and white.
There are a lot of ways to pick a theme.