Fixation involves small tremors and drifting off the fovea, and quick corrections . Riggs et al used mirrors and a contact lens to project an immobile stabilized retinal image . The result was image details disappeared and the image fogged over . Therefore, we need retinal changes to maintain an image. Differences in photopigment responses in cones are related to differences in wavelength.
Wavelengths have length, intensity and purity dimensions.
Percepts have hue, brightness and saturation, correspondingly. Saturation is the dominance of a single hue. White is desaturated, and has all wavelengths equally. Colour vision is synthetic - responding to combinations or syntheses , rather than analyzing into components, like audition. Hence, vision involves colour mixing, an additive process.