if you can see letters in the circles, you're probably a superhero
Human beings are trichromats, meaning we have three types of color receptors, cone cells, which determine the light ranges that our eyes have the ability to register (red, blue, green). This is more than most mammals. Most mammals only have two types of receptors (dichromatic), and thus are only able to discern between short and long wavelenghths. Each human receptor cone can perceive 100 graduations of color. The brain combines colors and graduations from each receptor. This translates into 1 million distinguishable hues for the typical human.
However, tetrachromats have four types of receptors often allowing them to, in some cases, see ultraviolet light or to make more precise distinctions between hues. The zebrafish is an example of a tetrachromat, as well as other teleost fish (most fish are teleost), birds, reptiles, and some arachnids and insects. The fourth receptor could exponentially increase the amount of distinguishable hues up to 100 million.
There have been a couple reported cases of human tetrachromacy, both involving women. Potentially 2-3% of women have an additional cone, inbetween red and green, which would allow for much greater hue differentiation. Ironically, the genetic anomoly which might cause tetrachromacy in women also causes color blindness in men.
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