Marron vs. brun
The French words 'marron' and 'brun' both translate to the English term 'brown'. However, they are applied in different contexts.
Marron
In French, 'marron' denotes a specific shade of brown, which is similar to the chestnut color.
Les cheveux de Marie sont marron.
(Marie's hair is chestnut brown.)
J'ai acheté une bourse marron.
(I bought a chestnut-colored purse.)
Les feuilles deviennent marron en automne.
(The leaves turn chestnut brown in the fall.)
Brun
'Brun' can refer to a generic brown color but it is also largely used to describe natural colors like hair, skin or eyes.
Il a les yeux bruns.
(He has brown eyes.)
La chaise est brune.
(The chair is brown.)
Mon ami est brun.
(My friend has brown hair.)
Summary
'Marron' and 'brun' are both used to describe colors in the brown spectrum. However, 'marron' usually refers to a specific chestnut brown while 'brun' can denote a general brown color and is often used to describe natural colors like hair, eyes or skin.