Im our eyes there are different types of cells that react to light. There are cone cells which allow you to see color, but are less sensitive to light. And there are rod cells, which are way more sensitive, but can’t preceive color. If the light is not bright enough, only your rod cells trigger and things appear black and white or grey-ish. That is also the reason why, as the saying goes, all cats are grey in the dark.
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