RavinRay
Explorer
B is already use for Blue.Print media uses CMYK - Cyan, Magenta, Yellow and Black (can't remember why black is K).
As for the bronze and brass dragons? My take: they are known for centuries, so it's easy to fit them in a fantasy setting that has a similar metallurgy history. Also, while a somewhat weaker case than for chromatic dragons, the five metallic dragons are also color-coded, but with a metallic luster to their scales: gold (metallic yellow), silver (metallic white), bronze (metallic brown), copper (metallic red) and brass (metallic pale yellow).