This grey shirt is either grey or an olive green (EDIT: According to another clothes-expert, this is a green heavy wool blend). It has a distinctive cut to the breast pocket flaps, similar to Dean’s S8 Marine Blue Shirt. It does up with buttons that match the colour of the shirt.
In Rock and a Hard Place (9x08), Dean wears this shirt at the end of the episode, when he and Sam say goodbye to Jody Mills, and Dean attempts to tell Sam about Ezekiel.
In The Purge (9x13), Dean wears this shirt at the end of the episode when he and Sam wrap things up at Canyon Valley and then also when they are back at the Bunker talking.
This might be the shirt that Dean wear’s in Mother’s Little Helper (9x17), or at least a dark blue version of the same shirt. It could also just be that the scene is lit differently. Dean wears the shirt for the entire episode.
In Hibbing 911 (10x08), Dean wears this shirt after Donna finds out about vampires and he and Sam can drop the FBI facade. He wears it until the end of the episode.
In Halt & Catch Fire (10x13), Dean wears this shirt when he and Sam are doing their best to prevent Delilah’s murder.