M: Looking for a word that is near in substance to monochromatic, but rather than color, describes style. As in, when I walked out of a dance tonight, I was kind of depressed because it wasn't a case of black or white people, but rather just a monochromatic style that I was seeing. Although in this case, depressed isn't really the right word, just kind of disappointed because I like variety.
G: I think you're reaching for a word that means low stylistic diversity, rather than anything about race or color.
A few possibilities, depending on the nuance:
Monolithic – Everyone seems to belong to the same aesthetic or cultural style. This is probably my favorite for your example.
Homogeneous – The most neutral and common term.
Uniform – Everyone dressed or presented themselves in much the same way.
Conformist or conforming – Suggests people are actively following the same trends rather than arriving there independently.
Monocultural – Usually refers to culture rather than clothing, but can work if you're talking about a shared subculture.
Cookie-cutter (informal) – Implies everyone looked as though they came from the same mold.
Samey (very informal) – "Everything just felt kind of samey."
If you're trying to preserve the metaphor behind monochromatic, there are also some more literary options:
From what you've described, I might phrase it like this:
"It wasn't about race at all. It was that the crowd felt stylistically homogeneous. I enjoy seeing a variety of aesthetics, and tonight it felt almost monolithic."
Or, if you want to keep the color metaphor alive:
"The room felt aesthetically monochromatic."
Even though "monochromatic" literally refers to color, English speakers often understand it metaphorically to mean lacking variety. Pairing it with "aesthetic" or "style" makes the intended meaning quite clear.
M: I'd say the best candidates were monolithic, monocultural, and one-note. Although none of those feels quite specific enough. Although that might just be because I haven't used them regularly. The only problem with monocultural is that that sounds like something begging to start up an entire internet blaze.
G: I think you're noticing something real about the semantics. None of those words is defined as "lacking stylistic diversity"; they're all borrowing from adjacent concepts.
A few more candidates, in roughly descending order of how close they get:
Your concern about monocultural is well-founded, though perhaps for a slightly different reason. Most listeners would hear it as referring to the people rather than the aesthetic, even if you intended the latter. You'd spend the next five minutes explaining that you weren't talking about race, nationality, or ethnicity.
The phrase that struck me as I read your example was actually:
"The crowd lacked aesthetic diversity."
It's surprisingly hard to misinterpret. It also captures your emotional point. You weren't wishing a different demographic had shown up; you were disappointed that you weren't seeing a broader range of self-expression.
Another metaphor that might fit your style of speaking is:
"The room had collapsed into a single aesthetic."
or
"The crowd had converged on one aesthetic."
Those convey the idea that a dance floor full of distinct personalities had nevertheless ended up looking stylistically similar.
One other word that came to mind is hegemonic, as in "a hegemonic aesthetic," but that's so loaded with academic and political overtones that I think it creates more distraction than precision.
If I were inventing a dictionary entry to fill the gap you're describing, it would honestly be monostylistic. It has exactly the morphology of monochromatic:
It's not standard, but I suspect most educated English speakers would understand it immediately without needing an explanation. Sometimes English's greatest strength is that it tolerates sensible coinages.