The rarest hair color is red. It's estimated that only 1-2% of the world's population has red hair.
Auburn hair, while less common than brown or black hair, is still more prevalent than red. It falls somewhere in between red and brown on the hair color spectrum.
Here's a general breakdown of hair color prevalence (most common to least common):
1. Black
2. Brown
3. Blonde
4. Auburn
5. Red