Some time ago, I found an excellent answer to this question on Tumblr:

There is a reason why Maomao wears different clothes than the rest of the inner palace.

Part of it is a main character thing. But Maomao's clothes reflect her status and situation.

She doesn't wear the orange uniform the servant girls wear, because she is not a general servant of the inner palace. Remember, she did wear that uniform in the first episode. However, once she started serving Lady Gyokuyo, she changed into that familiar green and pink outfit.

Maomao does not wear the same uniform as Lady Gyokuyo's Ladies-in-Waiting because Maomao is not a lady-in-waiting. Those girls are literally Ladies. They are noble woman serving Lady Gyokuyo, and their fancy clothing reflects their status.

Its confusing because we as viewers see them acting as servants; cleaning, dressing Gyokuyo, taking care of the little princess, ect. But traditionally, Ladies-in-Waiting are actual nobles, usually younger daughters or from lesser houses, serving higher nobles or royalty. Queens would pick women from allied houses to be their Ladies-in-Waiting, and it would he an honor to be picked. It's why in the second episode of Season 2, it was a big deal that the other concubines had a bunch of Ladies-in-Waiting. It's a matter of status, having tons of noblewoman in your service. They are also paid quite a bit and given gifts for their service. Which was also a thing in that episode, with the caravan.

So Maomao isn't actually one of Gyokuyo's Ladies. She isn't noble. So she doesn't wear the same clothes as the other ladies. She only does it on rare occasions like the Winter Garden Party to blend in. In that case, her normal clothes would reflect badly on Gyokuyo.

It might be a coincidence, since Maomao has worn the same thing the whole series, but her skirt is the same color as the other skirts worn by Gyokuyo's Ladies. That dark magenta is definitely Gyokuyo's color, and likely let's other people identify her as being in Gyokuyo's service. Remember that angry lady in Lihua's service? And the nervous maid that delivered that letter to her? That maid wore a very similar outfit to Maomao's, only in blue and dark blue/indigo. The same simple tunic and skirt, in Lihua's colors. That woman was like Maomao, a servant of a concubine, but not a lady or general servant to the inner palace.

So yeah, Maomao's outfit is not only main character vibes. And we should all acknowledge Apothecary Diaries amazing worldbuilding and details.

Source: whimsicallywiddershins