This shirt is a red and blue broad plaid. It does up with white snaps and is most likely cut in the western style (though that’s a guess at this point, based solely on the appearance of snaps and how the two often go hand in hand.) It’s got white to the plaid as well, but the really eye catching colours are the deep red and blue.
In Thin Lizzie (11x05), Sam wears this shirt after he changes out of his suit and he and Dean take Len with them when they attempt to track down the wife of one of the victims, who happens to be their prime suspect. He keeps it on until the end of the episode.
In Into the Mystic (11x11), Sam wears this at the end of the episode when he says goodbye to Eileen, and then back when he is at the Bunker contemplating.
In Rock Never Dies (12x07), Sam wears this shirt when they meet up with Castiel and Crowley in the hotel and check out Vince Vincente’s suite. He then changes into a “rock musician” disguise to continue to investigate the case.
In Ladies Drink Free (12x16), Sam wears this shirt when he goes to the university with Claire, so that they can talk to the victim’s best friend. He continues to wear it until Claire is cured.
In The Big Empty (13x04), Sam wears this shirt when they go visit Mia, the therapist. He continues to wear it until he and Dean have their conversation in the bunker at the end of the episode.