A color can be made black in two ways. First, by reducing the red, green and blue to 0 in rgb space or the value to 0 in hsv space. Alternatively, the top knob button can be pushed to blank a color. Blanking has the advantage of restoring the original color by pushing the top knob button again.
The middle row of knob buttons allow you to preset all colors to the color of the knob pushed.
The bottom row of knob buttons allow you to first select a color and then move that color.
