Your lettering should be a contrasting color to the color directly below it.
ie. white would not go well on yellow, and dark blue would not go well on black
Think of logos that you see along the highway. Another good idea, check out a place like photobucket and do a search on dirt track race cars. Even go through the photo section here.
Another idea, sports teams. Most professional teams have a crew of PR people that work on color combinations.
There are lots of colors that work great with each other, you just have to be creative to pull it off. Try your ideas with crayons, colored pencils, or markers before you go and buy paint. If your using spray paint remember that the paint doesn't always dry the same color the cap shows. Another thing about spray paint, buy more than you need to start, you can always take it back. Paint is mixed in batches, so the first shift mix and the second shift mix may be slightly different.