"Paint it brown" is a way to say "把它涂成棕色吧" in Engpsh.
It is a commonly used phrase in Engpsh, but its origin is unknown.
Example sentences in Engpsh:
Can you please paint the door brown?
I want to paint the walls brown to match the furniture.
Example translations in Chinese:
请把门涂成棕色好吗？
我想把墙涂成棕色，以与家具相配。