Should I always add a black paint into the mixtures?
The addition of a small percentage of black is just one way to experiment with the Interference Colors. Some artists always add black in their mixtures, while others rarely (if ever) add black. The addition of black intensifies the appearance of the main color (i.e. Interference Gold would have a more intense gold cast). Too much black however, can lower brilliance of the mix and create more of an iridescent effect than interference.