"You have a brown hat."
The word "you" usually equates to คุณ, but is also capable of equating to พวกคุณ. It usually depends on the context of the sentence to decide what is implied.
For this sentence, I agree that the singular คุณ is more appropriate.
Here is a link to another post describing examples where "you" is meant in a plural sense (พวกคุณ):