I believe it is a "near recognition symbol" to designate the unit the truck belongs to. When deployed (or training for deployment) bumper numbers are usually removed. Symbol shape, color and location are used to designate particular units or sub-units. A good exapmle on combat vehicles is the painted (or GLINT tape) dashes and chevrons on tanks, APCs etc.)
We own some trucks from the 42d Infantry Division that were used during their Iraq deployment and during the train up for it that have a variety of blue and white squares variously divided vertically, horizontally and diagonally.
Lance