I think greyscale and monochrome pics should have some kind of white/black/grey distinction for hair, and maybe clothes.
Different colors just give off a different vibe that needs to be searchable/blacklistable, even for greyscale posts.
You might even want to tag eye colors for certain art styles or characters with white irises, like byakugan or kose erika.
Things like black theme and white theme are plenty appropriate for monochrome posts, but not always applicable even when you'd want to search/blacklist hair/clothing colors.
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