You're probably right that hypothesis better fits my intention than theory. And they're not my hypotheses, they're common hypotheses on human perception from psychology: http://en.wikipedia.org/wiki/Opponent_process (also references trichromatic theory). Also, I'm well aware of the decreasing function of cones with decreasing light - that's why it's easier to distinguish shapes in the dark if you don't look directly at them: your retina is covered with cones at the center and rods around the edges (rods process vision without color).