"He buys a brown shoe."
Translation:Dia membeli sepatu cokelat.
The English sentence is a little bit strange with the indefinite article.
I would expect something like :
"He buys brown shoes"
"Dia membeli sepatu cokelat"
"He buys a pair of brown shoes"
"Dia membeli sepasang sepatu cokelat"
Buying a shoe sounds like you're buying only one shoe.
(only the left shoe or only the right shoe).