For truck front tires, we almost always use the standard foam that comes with the tire.
For truck rears, the longer the pin you are running, the softer the foam. With few exceptions, we run nothing stiffer than gray foam. Next softer is gray-and-white, then the standard foam. We don't run much gray, usually gray-and-white or standard. It depends on the dirt, compound, and pin you're running.
The consistant exception, front and rear, is Panther tires, which always seem to work best with a stiffer foam, usually either Panther's gray foam or 3-Stage.
Hope this helps.
Engineering Without Fear