I want to go into more detail on this, because I rewatched with this in mind and GOD do I love both details and costuming.
So the first time we see Wei Wuxian in this undershirt is when he wakes up in Cloud Recesses, right after everyone finds out who he is (and right after “I already knew it was Wei Ying.”)
This is also the first time you see Lan Wangji’s lack of a bottom layer.
Lan Xichen comes in and they discuss what Wei Wuxian had found out in the secret room, and Wei Wuxian quickly dresses so they can check out the forbidden room of the Library Pavillion. His bottom layer, normally either red or black until this point, is still white. Lan Wanji still has only two layers on.
And if you notice, Wei Wuxian’s layers aren’t quite sitting right on him. Both Lan Wangji and Lan Xichen have their robes tight against their chests, right up to their throats, but Wei Wuxian’s got that big white stripe almost screaming for you to look at it in comparison. In closeups you can also tell that it doesn’t sit smoothly against his chest like the costumes normally do in this show.
See that wrinkle? You NEVER see that in any other part of this show. Hell, the only time anyone has a single hair out of place is when Wei Wuxian’s trapped in the dungeon with that giant dog, and that was to show just how badly off he was in that moment. But that shirt don’t fit, bro.
And that’s what he continues to wear through the trip to the Burial Mounds. It’s less obvious later because he tucks it a bit tighter, but he still has that white bottom layer, and Lan Wangji is still missing a layer. Right up until Wei Wuxian uses himself as bait, as shown in the video. They finally change clothes when they’re on the docks on the way to Lotus Pier. Wei Wuxian gets a whole new outfit, and Lan Wangji suddenly has a bottom layer again – but instead of his usual white, it’s blue.
So why bother with this little detail? Why bother putting it in or spending a half hour of my life gathering screenshots to analyze it? Because of this passage (and others) from a bonus chapter of the novel, via Exiled Rebels Scanlations:
When the time came, Lan SiZhui had been waiting for quite a while, standing in the courtyard with Lil’ Apple reined, when Wei WuXian and Lan WangJi finally sauntered out of the house.
He wanted to tell him, Senior Wei, you accidentally wore HanGuang-Jun’s clothes again. But after some thought, he still swallowed the sentence.
After all, Wei WuXian wore the wrong clothes every couple of days. If he reminded Wei WuXian every single time, wouldn’t he die of fatigue?
And every time Senior Wei would wear it anyways because he thought it was too much trouble to change. Feeling that there was no point in reminding him anyways, Lan SiZhui decided he’d much rather pretend he saw nothing.
The novel makes a point of mentioning that Wei Wuxian constantly grabs the wrong underclothes, and that said underclothes are too big for him, but he doesn’t bother changing. And the showrunners paid attention and said, “Well, we may not be able to SAY that they get nekkid together, but we sure can show evidence for it!” And it’s that kind of attention to detail that makes this show so damn fantastic, and so true to the source material despite having to work around externally imposed rules of conduct.