A physicist would say: "in the dark all cats are black", because in the absence of lighting objects are not visible at all. But the saying does not mean complete darkness, and the darkness in the everyday sense of the word, i.e., very poor lighting. Exactly saying is: at night all cats are gray. Initial, permanent sense of saying that in low light, our eye ceases to distinguish the color of each surface appears gray.
Is this true? Whether in the dim light and the red flag and green foliage are represented equally grey? It is easy to verify the correctness of this statement. Who in the twilight was getting accustomed to the color of the items, he noticed, of course, that the color differences are erased and all things seem more or less dark gray: and the red blanket, and blue Wallpaper, purple flowers and green leaves.
"Through the lowered blinds, we read Chekhov ("Letter"), is not here sun was shining, it was twilight, so all roses in a large bouquet seemed to be the same color".
The exact physical experiments fully confirm this observation. If the painted surface to cover the weak white light (or white surface - painted weak light), gradually increasing light, the eye first sees just gray, without any color cast. And only when the light is amplified to a certain extent, the eye begins to see that the painted surface. This degree of coverage is called a "lower threshold color sensations".
So, quite literal and proper sense sayings (existing in many languages) that is below the threshold color sensations all objects appear gray.
Discovered that there is a higher threshold color sensations. When extremely bright light eyes again ceases to distinguish the shades of color: all painted surfaces are equally look white.