I assume that we are not, because it should have got more mentions if we were. Man has 4 kinds of light receptors. Rods and 3 types of cones. All cats are grey in the dark, because no cones work in the dark, and there is only 1 type of rods. In bright light, man is trichromatic because rods are dazzled, and there are 3 types of cones. But twilight? The sensitivity of rods does not match that of any type of cones. The maximum sensitivity of rods is here http://en.wikipedia.org/wiki/File:Cone-response.svg described as 498 nm, which is further (36 nm) from the maximum green sensitivity at 534 nm than the difference between green maximum and red maximum (30 nm). In twilight, man should have 4 types of receptors functioning with very different spectral sensitivities. 3 types of cones, and rods. Why do the pictures of cones and rods not form a full tetrachromatic picture and perception of tetrachromatic hues in twilight?