This is the hue of the background color, in degrees. The color's lightness and saturation are controlled separately; if lightness is 0 or 1, or saturation is 0, hue has no effect. The background color is also used as the outline color in Fill mode.
Note that hue is implemented differently for rings than for the background: for rings, hue is controlled indirectly via Color Speed, whereas background hue is an absolute setting. As a result, hue cycling for the background color requires an oscillator. For 360° cycling, set the waveform to "Ramp Up", the amplitude to 180, and the frequency non-zero.