Ruby can't handle white either. Her dress will be red. It's good because Dean doesn't know if he could handle seeing Sam wear white. Lucifer visited far too many horrors on them in that suit. To be clear, the issue isn't that Dean interacts with Sam and sees Lucifer; it's very obvious that Sam is not Lucifer because Lucifer was never that good an actor. It's that Dean has problems looking at Sam because he knows he failed him in every possible way. Lucy believes with time that the boys will get there and she refuses to accept any other reality.

"It wasn't a hard decision. She did everything that I should have." Stay with him, Dean means. At the same time, he is very aware that Lucifer probably would have killed him eventually. Dean's brow furrows a little while Sam talks about reaching past that barrier. "If I remember right, there was another you there at the same time there was another me. I've seen enough sci fi movies that I'm guessing that had something to do with disrupting something or some shit like that."