Sam really likes his blue stripes. This shirt is light blue, white, and pale yellow striped. The pale yellow is a bit hard to pick up on in most shots, but you can usually make it out on the CU. The stripes are even in size.
It only appears in Something Wicked (1x18), when Sam wears it for the entire episode. He only takes it off when he wears a suit for their first visit to the hospital, and when he is in their motel room.