Mixing coloured light, where combining colours moves toward white — how screens work.
Additive colour mixing combines light of different wavelengths; red, green and blue light combined at full intensity produce white, which is the basis of the RGB model used by digital displays.
Additive mixing (light) and subtractive mixing (paint/pigment) behave in opposite ways — mixing all additive primaries gives white, mixing all subtractive primaries gives near-black.
A phone or TV screen produces white by combining red, green and blue light at full brightness.
Additive vs subtractive mixing is the core reason screen colour and paint colour don't always behave the way people expect when combined.
Additive mixtures obey simple arithmetic: the tristimulus values of a mixture of lights are the sums of the tristimulus values of its parts. A display is characterised by measuring its three primaries and its white with a colorimeter or spectroradiometer, after which any mixture can be predicted. The sums must be taken in linear light. sRGB code values are gamma-encoded, so they have to be decoded before adding and re-encoded afterwards.
| Light | Result | Relative luminance |
|---|---|---|
| Red #ff0000 | #ff0000 | 0.2126 |
| Green #00ff00 | #00ff00 | 0.7152 |
| Blue #0000ff | #0000ff | 0.0722 |
| Red + green #ffff00 | #ffff00 | 0.9278 |
| Green + blue #00ffff | #00ffff | 0.7874 |
| Red + blue #ff00ff | #ff00ff | 0.2848 |
| All three #ffffff | #ffffff | 1.0000 |
Red (0.2126), green (0.7152) and blue (0.0722) sum to 1.0000, which is white. Every mixture is lighter than either of its parts, the signature of additive mixing. Red and green light make a yellow with 0.9278 of white's luminance, a result no red and green paint can approach.
How this was calculated: WCAG relative luminance of each full-strength sRGB combination; a sum of lights is a sum of linear channel values, so the luminances add exactly.
A gradient or a blur is computed directly on sRGB code values.
Averaging encoded values is not averaging light, so the midpoint of a red-to-green gradient comes out as a dark, muddy olive. Blending in linear light gives the lighter yellow that real overlapping beams produce.
Stage or architectural lighting mixes red, green and blue LEDs to make white.
The beams sum to a white that looks right on a wall and renders coloured objects poorly, because three narrow bands leave gaps in the spectrum. Fixtures add amber, lime or white emitters to fill them.