Is there any major difference between 'czapka' and 'kapelusz' in meaning? Or are they interchangeable?
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).