* بني (banī): This is a very common and widely understood word for brown. It's a good general-purpose choice.
* أسمر (ʾasmar): This word often refers to a swarthy or tanned complexion, and can also be used to describe dark brown things.
So, while بني (banī) is likely the best option for most situations, the context matters. If you're describing dark brown skin, for example, أسمر (ʾasmar) might be more appropriate.