Too many pinkies needing to be rescued in a row for me. Lol. Black, blonde and red make sense. Those are much more normal.
Fishman island, Dressrosa, and WCI all have arcs inbetween them which means they're not in a row. How are those hair colors more normal than pink in the world of One Piece?
Green I assumed that since that was often used as a specific Zoro descriptor we wouldn't see it that often.
Monet, Bartolomeo, Kokoro, Chimney, Sugar, Sandersonia and Gaimon all have green hair.
Red is more of a Shanks unique thing. Orange is Nami.
Then why do Calgara & Kid have it? There are a couple of characters with orange as well.
I just mostly was hoping that with Big Mom and some of her kids winding up with pink Sanji's sister wouldn't also have pink. Just makes Reiji even less unique.
There is no character with a unique hair color.