I found this on Yahoo Answers...basically brass is more yellow and bronze is more brown...
Bronze vs. Brass
1. Bronze is the familiar brownish color whereas brass is the more greyish greenish, bluish. Both will weather to the fine verdigris patina without maintenance.
2. They differ in the amont of metals used in the amalgam. Brass is a combination of copper and zinc while brass is a combination of copper and tin.
3. Bronze items are four times more expensive than brass.
4. Bronze is much stronger and more corrosion resistant than brass.
5. Bronze is harder and more abrasion resistant than brass
6. As brass deteriorates, it creates an oxide (a grey white powder - zinc oxide). Zinc oxide is acetic and will attack the lignum in wood. Once the lignum is gone the wood fiber is open to rot
7. Brass melts at lower temperature and therefore use less energy to melt. It machines and polishes much easier than Bronze and therefore the price to make a fitting from it is lower. Also, it goes away much more quickly and therefore the customer will have to purchase replacement parts much sooner.
8. Bronze is richer, more golden in color than brass which is usually a yellow color. If there is any doubt in your mind, try an easy test. Using the smallest drill bit that you have, drill a small hole in an unimportant area of the fitting and look at the metal turnings that come out. If they are long and stringy, the fittings are probably Bronze. If the turnings are small (like snow flakes), then the metal is probably brass and even more probably a leaded brass.