Bronze can take many different colors depending on the relative amounts of copper and tin, and how the bronze is treated. It is also possible to create many different patinas artificially. There is some helpful information on these sites:
http://www.grayson.edu/grayson/homep...rks/bronze.htm
http://www.whiteriverfoundry.com/bro...ina_basics.htm
http://www.mckaylodge.com/tylerdavid...technical.html
Bronze is distinguished from brass metallurgically based on the metals included in the alloy -- brass is an alloy of copper and zinc. Just looking at an object, it can be very difficult to distinguish between brass and bronze.
Ian.