I agree that a language convention in which pronouns map to hair color doesn't seem great. The people in this world should probably coordinate on switching to a better convention, if they can figure out how.
-But this convention as given, a demand to be referred to as having a hair color _that one does not have_ seems outrageous to me!
+But taking this convention as given, a demand to be referred to as having a hair color _that one does not have_ seems outrageous to me!
It makes sense to object to the convention forcing a binary choice in the "halfway between two central points" case. That's an example of genuine nuance brought on by a genuine complication to a system that _falsely_ assumes discrete hair colors.