5-1 marron -vs- châtains

• marron brown
• is invariable
• does not agree with gender or number with the noun it modifies/describes.
  

• châtains brown (hair)
• almost exclusively used to describe hair color

ex;
Mon neveu a les yeux marron. My nephew has brown eyes.

-vs-

Ma nièce a les cheveux châtains. My niece has brown hair.