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.