They are totally different, so for a Polish learner of English the idea that one word could be used for both of them is... well, let's say "strange" ;)

You can take a look here.

Generally, "brunatny" is rather rarely used. It's a dark brown colour. It mostly works in "węgiel brunatny" (brown coal), "niedźwiedź brunatny" (brown bear), it may also be figuratively associated with fascism (NSDAP had shirts of this colour).

Ą. It's something between 'ow' and 'on'.

Okay, added.