You're talking about auto focus and it's true that focus detectors in modern cameras need some contrast to work. Cameras use lightness/color for auto exposure

You're talking about auto focus and it's true that focus detectors in modern cameras need some contrast to work. Cameras use lightness/color for auto exposure (and auto white-balance). If you shoot a picture of a white sheet of paper with auto exposure on, cameras will typically try to adjust exposure so that it comes out looking gray-ish.