The Humans With Super Human Vision (Discover Magazine)
"Researchers suspect, though, that some people see even more. Living among us are people with four cones, who might experience a range of colors invisible to the rest. It's possible these so-called tetrachromats see a hundred million colors, with each familiar hue fracturing into a hundred more subtle shades for which there are no names, no paint swatches."
The Dimensionality of Color Vision in Carriers of Anomalous Trichromacy (Journal of Vision)
"Some 12% of women are carriers of the mild, X-linked forms of color vision deficiencies called 'anomalous trichromacy.' Owing to random X chromosome inactivation, their retinae must contain four classes of cone rather than the normal three; and it has previously been speculated that these female carriers might be tetrachromatic, capable of discriminating spectral stimuli that are indistinguishable to the normal trichromat."
Tetrachromatic Color Vision (UC Irvine)
"The term 'tetrachromacy' describes the physiological possession of four different classes of simultaneously functioning retinal photopigments (also called 'weak tetrachromacy'). From an empirical standpoint, tetrachromatic color vision (or 'strong tetrachromacy') additionally requires demonstrating that mixtures of four independent appropriately chosen primary lights will simulate all distinctions in appearance possible in visible color space."