The lettering on the back is actually "dropped out" from the background tire tread with a slight outline, meaning it picks up the color of the shirt and keeps the back to a simpler (less expensive?) one screen/color black print. The lighter the shirt, the higher the contrast.
It can be done in white (or any color) with an additional screen, or the lettering can be placed adjacent to the tread, depending on what the decision makers decide.
