This shirt has tiny buttons in comparison to the usual size of Dean’s buttons. The shirt itself is a very light green/off-white colour. The buttons are dark brown or green.
In Just My Imagination (11x08), Dean wears this shirt when he and Sam go to investigate the death of Sully’s pal, Sparkle. He wears it for the rest of the episode, except for briefly, when he wears a child-psychologist disguise in order to see Sparkle’s body.
In O Brother Where Art Thou (11x09), Dean wears this shirt when they go to Crowley’s throne room to recruit Rowena.
In Beat the Devil (13x21), this MIGHT be the shirt that Dean is wearing in Sam’s dream at the beginning of the episode.