Castanho vs. marrom
While both castanho and marrom represent the color brown, their usage depends on the specific variety of Portuguese (Brazilian vs. European) and the context of the object being described. In Brazil, there is a strict division between biological features and inanimate objects, whereas in Portugal, one term is dominant.
Castanho
A1In Brazilian Portuguese, castanho is reserved almost exclusively for body features, specifically when describing the color of eyes, hair, or fur. In European Portuguese (Portugal), castanho is the standard, universal word for brown used for everything, including objects and surfaces. It is derived from the word for chestnut.
Ela tem lindos olhos castanhos.
(She has beautiful brown eyes.)
O meu cabelo é castanho claro.
(My hair is light brown.)
No outono, as folhas ficam castanhas.
(In autumn, the leaves turn brown (Common in European Portuguese).)
Ele pintou o cabelo de castanho escuro.
(He dyed his hair dark brown.)
A cor de seus olhos é castanho esverdeado.
(The color of his eyes is hazel brown.)
Marrom
A1In Brazilian Portuguese, marrom is the general term for the color brown used for clothes, furniture, paint, manufactured objects, and surfaces (like dirt or wood). It is rarely used in Portugal, where it is considered a foreign usage generally specific to Brazilian speakers.
Eu comprei um sofá marrom para a sala.
(I bought a brown sofa for the living room.)
Você prefere a bota preta ou a marrom?
(Do you prefer the black boot or the brown one?)
Os ursos marrons são grandes e fortes.
(Brown bears are large and strong.)
A caixa de papelão é marrom.
(The cardboard box is brown.)
Misture vermelho e verde para fazer marrom.
(Mix red and green to make brown.)
Summary
The main difference lies in capitalization on the object and region. In Brazil, use castanho strictly for hair and eyes, and marrom for all other objects and materials. In Portugal, use castanho for almost all situations.