Point at the wall to read the color of any spot.
How does this work?
Your eye has three kinds of color sensors, tuned roughly to red, green and blue light. Every color you see is just how strongly each of the three is lit.
Light adds. Each spotlight lights up one sensor, so red, green and blue are the primaries of light. Where beams overlap the wall gets brighter:
- Red + green = yellow
- Green + blue = cyan
- Red + blue = magenta
- All three = white; no light at all = black
Paint takes away. White light hits the paint, and the paint soaks up part of it. The best paint primaries each soak up exactly one of the three: cyan soaks up red, magenta soaks up green, yellow soaks up blue. Where see-through paints overlap, more light is gone:
- Cyan + yellow = green (only green light is left)
- Cyan + magenta = blue
- Magenta + yellow = red
- All three = black; no paint = the white wall
The overlaps in Light mode are the paints in Paint mode, and the other way around. Turn every slider to 255 in each mode and compare.
Why do art classes say red, yellow and blue? That rule is older than our knowledge of the eye. A painter's "red" leans toward magenta and "blue" toward cyan, so it works roughly, but it can't mix a bright magenta or cyan. Printers use cyan, magenta and yellow ink for exactly this reason.
Each number from 0 to 255 is how strong that light or paint is. Three of them give 256 × 256 × 256 = 16,777,216 possible colors, and every screen makes them from tiny red, green and blue lights.