As for Reimu, I'm probably going to try going with a wine-red and white or off-white. While the in-game depictions of Reimu usually tend towards the more basic red and white, I've seen artwork done by other artists that give Reimu a slightly more "realistic" palette with darker reds. Instead of white, I could probably go with cream with that sort of palette.
Depending on the exact colors and how distinct it actually is versus Austria, it could be pretty nice. Shame there's no way to have Marisa truly be the Black/White either. (Well, without conflicting with City State coloration.)
So in line with my comment that I'd test playing against certain civs, I decided to test myself along with Yumemi AI.
I played as Remilia on King, without building a single world wonder or taking a second city in any way. My only bonuses were from buildings, religion and national wonders. This was partially as a challenge and partially to see what the AI did if left with free reign over all world wonders.
If Korea/Babylon/Poland/Maya are the very top tier for Firaxis' civs, then Eirin is their equivalent in terms of the Touhous. She was an absolute monster the entire game. She was tech leader and wonderspammed to all hell all game. I won on Science while BEHIND her on tech. (She had actually hit Future Tech several turns before I even
started with the spaceship part rush.) I'm not entirely sure she's overpowered so much that she just outclasses all her competition in a way that honestly matches how I personally interpret her character. Perhaps not the best indicator for balance, but this is honestly the sort of performance I'd want to see from Eirin as a leader. (And it's the sort of performance I'd like to see Yukari be capable of as well.)
Here's Eirin's wonder list at the end of the game.
Regarding less established civs:
Yuuka: (note that I still have the super early version that doesn't even have proper code to render sunflowers.) She seems to have done fine, although I never really saw any indication that she really uses any of her uniques. I admit the fact that the version I've been using being so old means I don't really pay quite enough attention to what she does. She does seem properly aggressive, although I kinda feel she should be sort of aligned more for one sort of victory. As it stands her uniques seem to almost have been taken from three different civs, even if they do all fit her and somewhat align in terms of how to use them. The whole thing with the sunflowers removing food seems like adding challenge where there shouldn't really be one, and the AI probably won't be able to ever handle that. I don't really know what to suggest though, Yuuka's a hard subject to work with.
Shinki: Since it's been mentioned here, I made sure to keep an eye on her progress. That said, Eirin kinda ruined any shot she had at being a runaway.
Here's a screencap of what scraps she picked up, and also her stellar relations with everyone. It doesn't really look like she was looking at tourism at all to be honest, although her plans of conquest didn't get her very far either. I don't know what to make of this, and honestly I'm not sure if any judgement could even be valid with a sample size of one. (Although I admit she's easy to initiate a player lead runaway with.) Also worth noting is that she started in grassland forest. And the Kappa were thrown into the arctic.
Yumemi: For a long time she was more or less neck and neck with the Kappa, and both of them are definitely very strong in the early game. One again, Eirin kind of wrecked them, but as the
Science Victory screen can show, she wasn't doing too bad. Honestly, if I had left it going, it'd have probably been her racing to space versus whatever the lunar juggernaut was planning to win with. As for whether or not the AI knew how the assistants work, I can't say. She was doing fine on tech early on though, and I'd say it's at least a decent sign that it figured it out?
Keine: She didn't really seem to be anything special really. She took a city off the Oni early on, but lost it again after some time and then never really amounted to anything. She certainly did nothing worthy of note.
Suika: I just want to comment that I've never really seen Suika be a frightening Oni horde, to rival Ghenghis or Attila (or Utsuho). She always seems to just be a relatively passive civ that's middle of the road in terms of performance and rarely seems to accomplish much. I might have to try a conquest game as her and see how that goes. As it stands though, I've never seen the AI be very powerful with her.