I love this post!!! He has history with odd clothing and like you say I'm sure it's just what he can find that fits (and perhaps a little bit of he doesn't deserve anything nicer or prettier - but you do, Sam. You do!!)

I still think that this is his spare jacket, the one he doesn't really like wearing but has no choice because his others are either covered in something gross that he can't wash out (yet, there will research!) or are in the washer as we speak. Maybe Dean bought it for him as a joke, and Sam wears it just to spite him! Or maybe Sam was temporarily injured when Dean bought it, and couldn't really see the colour clearly, and now that he can, he wears it on purpose!!!