But those are your guesses or interpretations - not what's actually in the movie. There is no actual explanation for why his beard is suddenly dark in the movie.
My point is this clear/obvious change in look spoils the surprise (at least for people who are paying attention) because he looks so different to how he looked on Ah-choo a few scenes previous (the only other alternative thought is "Luke dyed his beard/hair... to go into battle?!")
Thank goodness Poe got the "whole point" because Luke didn't actually TELL anyone (including Leia when he's right in front of her) and everyone in the Resistance was just kinda standing there watching him outside and going "Oooo..." before Poe figured it out.
The way it plays out in the movie (had Poe not - purely by chance - figured it out,) they could have all simply stood there gawking the whole time and Luke would have disappeared/died... and it would have all been for nothing; everyone then gets killed.
Right?
You have to be careful with the "you missed the point" type stuff when it comes to TLJ - because a lot of it makes no sense if you actually think it through, even in a very basic way.