When the neuron is stimulated and an action potential results the inside of the neuron briefly becomes positive relative to the outside of the neuron. When this occurs the neuron is said to have been depolarized. Hue, Saturation and Brightness Hue is most closely related to the wavelength of the stimulus. Different colors have different hues. Saturation is related to how much white content is in the stimulus.
Monochromatic hues are very highly saturated.
The least saturated color is white. So for example a pink would be less saturated than a spectral red but more saturated than white. Brightness relates to the amount of light coming from a source or being reflected from an object. Strictly speaking the amount of reflected light is called lightness. If you will look at the representation of an action potential you'll note that after the potential becomes positive it drops rapidly and for a brief time is more negative than it was at its resting potential.