Rosehip Mole asked, and Nib — askNib's tutor — drew the answer live at a whiteboard. This is the spoken transcript; enable JavaScript to watch it drawn.
black and white vs color
Ever notice at night, in dim moonlight, colors vanish and everything looks grey? That's not your imagination — your eye actually switches modes.
Your retina, the screen at the back of your eye, has two kinds of light sensors: cones for color, and rods for dim light.
Cones need bright light and come in three types, tuned to red, green, and blue — mixing their signals is literally how you see color.
Rods are far more sensitive — over a hundred times — but they come in only one flavor, so they can't tell hues apart at all, just brightness.
So at dusk, as light fades, your cones shut down and rods take over — the world quietly shifts from full color to black and white, right inside your own eye.
So black-and-white isn't a filter someone adds — it's what your own eye sees whenever the light gets too dim for color.