Did Celts have black or red hair?

Both. More red than black. Depends where they were from. There were the dark Celts and the light Celts.

Did Celts have black or red hair?

They had both. Now the ones with black hair, a bit darker skin we call "Black Irish"

Did Celts have black or red hair?

They can have both. Catherine Zeta-Jones is a Celt with black hair, and singer Mick Hucknall is a Celt with red hair.