Why do we say 'dog whistle' but 'dog's lead'; 'horse box' but 'horse's bridle'?
Yes, they are compound nouns. A "dog whistle" is a type of whistle, while a "horse box" is a type of transport.
As for the "team colours" these refer to a specific set of colours used by a particular sports team. Using your Norwich City F.C. example, that organization as a collective whole (and not a single person or two) has owned those colours since the 1907-1908 Southern Football League season.