I see the symbolic purpose of it being the blue saber since it’s specifically the one he threw over his shoulder at the start of the movie, and that Kylo wanted so badly in TFA, and I get that blue looks more aesthetic against red on Crait… But wishing Luke had projected with his green saber is my one guilty fandom-menace-tier sequel opinion.
I got the impression he was projecting as Ben remembered him in order to encourage Kylo to not think too much about it, but the blue saber is a big giveaway. Obviously Kylo was really, really, very angry, so it’s totally plausible that he didn’t notice, or assumed Luke was using a different blue saber, but it seems unnecessarily reckless on Luke’s part considering he went to the effort of otherwise looking like his younger self, hair and robes and all.
And now that the Mandalorian is giving us imagery of Jedi Master Luke - the “legend” that Luke is intentionally trying to leverage at the end of TLJ - having that green saber, it feels like a reprise of it on Crait would tie better symbolically into Luke’s franchise wide arc. After all, even if it isn’t the exact same saber he rejected at the start, I think seeing Luke accept a lightsaber again is essentially the same.
I think using both would’ve been odd, though. We’ve never got the impression that Luke duel wields like Ahsoka, and it would’ve struck me as quite over the top.