Green caps on tire valve stems usually mean the tires are filled with nitrogen instead of ordinary air. Car dealers, tire dealers and repair shops have touted nitrogen for several years as a better alternative to air based on claims that nitrogen doesn't leak as much, so tires stay fully inflated longer. The catch is that topping off the nitrogen typically costs from about $5 to $10 per tire, and the initial charge for filling the tires can be much higher, so it's also a revenue source for service outlets.
