aux cheveux poils de carotte

Question

What is the need for having two words for hair in this expression?

Accepted Answer

Hi Sydney,
Maarten beat me to it, I did spot this yesterday and asked for it to be corrected before answering you.
The colour poil-de-carotte ( invariable) is used to avoid saying 
roux
or
rouquin 
it means red-haired/ ginger.
 
Bonne Continuation !

Answer

This is 'poil de carotte' as an adjective (expression) for 'ginger/red' - qualifying the noun cheveux. According to wordreference and Larousse though it is invariable, does not take agreement, and should not be 'poils de carotte'.

https://www.larousse.fr/dictionnaires/francais-anglais/poil_de_carotte/663125

https://www.wordreference.com/fren/carotte

Answer

who knows