Aluminum housing and Brass back plate.

MIROAK-C 斑紋ガス青銅色(BLUE)
Aluminum housing and Copper back plate.

Both brass and copper are colored using traditional techniques by Momentum Factory Orii.
Brass is GREEN.
Copper is BLUE.
Each piece is handcrafted by craftsmen, so the patterns and colors are different.

Brass and copper are very similar in sound.
You can’t tell unless you compare them, but the copper is a little crisp, low-density, and thicker in the midrange, while the brass is a little more vivid in the mid-high range.