This operator computes a blend (or a mix) of its two color inputs, and outputs the resulting color.
If the Bias parameter is not adjusted, or is just clamped to the unit interval, the blending function is linear and the operator becomes a special case of the Mix operator, only catered to color data. The colors will be extrapolated linearly if the bias is used "as is" and falls outside the zero to one range.
A cubic function is used to interpolate the values if you choose the "Ease In/Out" option, in which case this operator behaves very much like the Smooth operator.
If you choose the "Cardinal Spline" option, the two colors are interpolated non-linearly and the result is similar to using a Spline operator with its "u" input acting as the bias, and the primary and secondary colors piped into the first/second and respectively third/fourth "key" inputs.
The Color Blending Space menu allows you to blend RGB colors in HSV space before converting the result back to RGB (if you choose the "HSV" option). With the default "RGB" setting, incoming RGB and HSV colors will be thrown into the mixer without any pre or post conversion.
This operator is meant mostly as a convenience tool for attaching color to patterns and to other operators that output an "amount" in the zero to one range. For example, you would pipe the output of the Rings operator into the "bias" input of this operator and vary the two colors for a look that suits your needs.