In a move atypical of Dean, he acquires a very loud plaid shirt in S8. This shirt looks to be flannel and cozy. It’s white, blue, red, and yellow – with red and yellow being the more prominent colours. It has two breast pockets and does up with snaps (again, atypical of Dean’s taste.)
In Everybody Hates Hitler (8x13), Dean wears this shirt the morning after Sam and Aaron are attacked by the necromancer at the library. He keeps it on until the necromancers are defeated and disposed of.
In The Great Escapist (8x21), Dean is wearing this shirt when he and Sam arrive at the hotel in Colorado in the hopes of finding Metatron. He keeps it on until Sam’s ice bath and then changes out of it (presumably, because it got wet.)
In Road Trip (9x10), Dean wears this shirt when he, Cas, and Crowley, leave to track down Gadreel, who has stolen Sam.
In Blade Runners (9x16), Dean wears this shirt at the beginning of the episode when he is trying to get in touch with Crowley.
In Fan Fiction (10x05), Dean wears this shirt during the performance of the play until the end of the episode.
In Halt & Catch Fire (10x13), Dean wears this shirt at the beginning when he and Sam are in the Bunker, and then again when they burn the truck that they believe is the anchoring object for a ghost.
In The Prisoner (10x22), Dean doesn’t wear this shirt, but we see Elden Styne throw it onto the pile of books that he then covers with gasoline.
In O Brother Where Art Thou (11x09), Dean wears this shirt from his first scene until they meet up with Crowley. He changes before they go to Crowley’s throne room to recruit Rowena.
In Beyond the Mat (11x15), Dean wears this shirt to the wrestling match after the funeral, and keeps it on until the end of the episode.
In There’s Something About Mary (12x21), Dean wears this shirt when he and Sam get back from seeing Eileen’s body at the morgue. He wears it until they find the microphone in the Bunker and lay their trap for the BMoL.
In Unfinished Business (13x20), Dean wears this shirt at the end of the episode in the Bunker when Sam confronts him about his behaviour as of late.