The main difference between these, is normalization, and the psyche of ordinary people. Brunette is "normal" in the sense that, in fiction, anything is possible, so why Brunette? Darker hair is most often for the 'main' character, who's there for the MAIN main character (who gets to unravel through the series, and give the story actual depth). The 'main' character is a stand-in for self inserts: The sad otaku wishing to find love, the one person who feels left out, the one having family issues, the character being tied more heavily to reality. Almost npc-like going through the series as a reactive,1-way street, camera lens kind of character than a fully fleshed out one. Blonde is also a natural IRL hair color much like brunette, but often gets the unrealistic "huge boobs/ natural talent" or unfair "bimbo/ himbo" cliche personalities tagged to their forehead, even throwing in villainous, or b*tchy, attitude at times.. and it makes blonde seem more fictional, and unrealistic when blondes are in fact just as real as brunettes; The same way redheads are presented as raging, passionate, driven, and often warrior queens, and kings of street brawls. The fact that these three aren't on equal footing in fiction as they are in reality is a bummer. Tropes that should have been broken a long time ago.