This shirt is a rich brown colour with lighter brown buttons. It’s got a nearly invisible left breast pocket that has no pocket flap.
In The Big Empty (13x04), Dean wears this shirt at the beginning of the episode when he is in the Bunker, and then again later when they dig up the grave to see if they’re dealing with a zombie.
In Unfinished Business (13x20), Dean wears this shirt in the motel at the beginning of the episode. He changes out of it before they go after Loki.