In Blade Runner 2049, after Officer K had reached home, he talks to Joi via holograms.
We can see her dress changes frequently during the conversation. Why so?
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As pointed out so poignantly@, Joi is changing her clothes to try and suit K's mood. Joi is a pleasure bot, she's designed purely to make sure her 'owner' is happy. She does a damn good job at it.
One of her means of achieving this is by 'looking' as pleasing to K as she can. She reads his moods and emotional cues#, and adjusts to suit.
At this point, K's mood is tarting to go off the radar, he's starting to drift from his baseline, this is foreshadowing as Joi's connection to K is incredibly deep.
You can see that when she goes from the white outfit to the black one$, it is synced to her change of stance and demeanor; she's changing her look to suit her intent. Yes, this is ultimately to suit K's mood.
The out-of-universe reason for this is to give us clues about her being. She's an incredibly advanced piece of technology. This ties into her ability as a chameleon and gives us a bit of foreshadowing of her true nature and later events with Mariette.
The other part of this is that it shows us how unreal she truly is. It's blatantly obvious that she's a machine. Yet, why do we still feel her reality and come to believe that she truly loves K? That's why I think she was one of the most well-written characters!
# Yes, he does have emotional cues, albeit incredibly subtle.
$ As a dude, that is the extent of my fashion knowledge