Colors of old bronze artifact are mainly due to patination which is environment dependant. Bronze compositions are varied, even within a culture. There are recipe for 2-9 (and 9+) components alloy for Siamese bronze. Some of them go closer to brass and some of them contains gold. So you can find a full spectrum of red to yellow color. And evenmore complicate, different alloy element makes different patination rate. There 's a microscopic analysis for fake patination but it 's quite difficult to determine origin/composition/age of bronze by their colors.
