How smart you are to think this through! First, is there a signature color for your logo or company? If so, consider that color. If corporate identity is not an issue, go with your favorite color. If it is a memorable color -- bright red or bright blue, for instance -- you will be remembered for the color of your truck. It will be a part of your personal brand.
No matter what color you choose, make sure your truck is always clean and sharp both inside and out. A messy truck sends a bad message about your attention to detail.