How can copper oxide produce yellowish-green enamel?

In enamelwork, a yellowish-green color can be produced from the black oxide of copper (cupric oxide) by increasing the proportionate amount of red lead within the flux mixture.1

While the black oxide of copper is the base coloring agent, the final hue of many enamels is determined by adjusting the proportions of the flux components rather than changing the quantity of the oxide itself. By modifying the chemical composition of the flux—which is a compound of flint or sand, red lead, and soda or potash—craftsmen can achieve different color variations from the same metallic oxide.1