Use the following trick to change your truck's color without selling and
buying it again. Enter multi-player menu and start a LAN game, with your
PC as the host. At the multi-player game setup screen, go to the menu bar
at the top of the screen and select the player profile that contains the 
vehicle to have the new color. Select that vehicle and a color select menu
will appear. Select the desired color and exit multi-player mode. When you
start career mode again, the color you selected will be on the vehicle.