Brass is the name given to those copper alloys that contain zinc as the major alloying constituent. The brasses are broken down into a series of groups classified by the crystal makeup of the copper alloy. As more zinc is added the crystal structure of the copper alloy changes. This change affects the color and the forming ability of the metal.
The term brass is often used to reference the color rather than the alloy. For instance, you can polish bronze nautical fittings to a lustrous golden yellow, a color most associate with the term "brass".
Similar but opposite — when weathered and patinated, many copper alloys will exhibit similar visual characteristics. The dark brown or green patina which forms on the surface of copper alloys disguises the metal underneath its protective patina. When people call a material "statuary bronze", they are referring to the name given to describe a range of bronze-color tones produced by chemical enhancement on various copper alloys, both brass and bronze alloys.
For the reasons above, selecting a copper alloy for your project should be based on how the metal will perform in any given environment.
Fabricators who work with the metal and architects who design with it have, over the years, referred to the copper alloys as bronze if their color ranges are in the darker brown tones.