[JKCalhoun]

Interesting. In nature, I suppose I am most aware of a gradient from a saturated azure sky above to a much whiter sky on the horizon. This would seem to be a trivial saturation gradient.

For a spot color (from a gel covering a light) the light diffuses further from the center of the projected light — two spot colors (with different gels) then, next to each other, would give a kind of gradient from one color to the next as you walk a line from the center of one light to the other.

I wonder what the closest analog to this is algorithmically?

I guess where I am going with this is: is there precedent in nature as to how gradients are supposed to work (and therefore an analog which we should try to model) or are we going strictly on how the human eye perceives color and what algorithm we think "looks" right?