Vehicle colors can be viewed in the image below:

(image hosted on SA-MP wiki)

To assign a vehicle color in-game, you can...
1. Use the /settings menu and navigate to Vehicle Preferences and select Saved color.
2. Use the /cc (/carcolor) command like this: /cc 69 69
3. To save the color permanently, you can also use: /cc 69 69 yes (final parameter being "yes")