...remember how Mary's hair seemed soooo weirdly brown in
When the Levee Breaks? ( In Bobby's panic room during Sam's detox delusion). Even to the point of me questioning if she was meant to be Mary??? Got a bit muddled for a minute, you know? Last night I rewatched WIAWNSB (or a similiar combination of those letters) and the scene where Dean first goes to the childhood home to find Mary, her hair looked really blonde when the light was behind her , ie. opening the exterior door. It was more brown and without the blonde highlights at other times, ie talking to Dean, turning off the light before saying goodnight to Dean and return to bed.
I reassure myself that the actor is unlikely to have changed her hair colouring over the space of scene, even if filmed over couple of days. Sooo, I'm thinking it's an issue of lighting which altered her look and hair colouring. Maybe it was intentional, and I just missed the memo!