Additive and Subtractive Color Mixing

Color reaches your eye as light - a spectrum of wavelengths. There are only two fundamental ways to control which wavelengths arrive, and they are mirror images of each other:

Additive mixing starts with no light - black - and adds colored light. Each source you turn on contributes its wavelengths; the more you add, the brighter and ultimately whiter the result. This is how anything that emits light makes color: screens, projectors, stage lighting, the sun through stained glass.

Subtractive mixing starts with full light - white - and removes parts of it with colorants that absorb. Each ink, dye, or pigment subtracts some wavelengths; the more you add, the darker the result, heading toward black. This is how anything that reflects or transmits light makes color: paint, ink, dye, filters, the physical world.

The additive primaries are red, green, and blue (RGB). Shine all three at full strength on the same spot and you get white. Overlap them in pairs and you get the bright secondaries: red + green = yellow, green + blue = cyan, red + blue = magenta. Drag the three lights below from off (black) to full (white) and watch them sum.

The subtractive primaries are cyan, magenta, and yellow (CMY). On white paper, each acts as a filter removing one third of the spectrum: cyan absorbs red, magenta absorbs green, yellow absorbs blue. Overlap them in pairs and you get the subtractive secondaries - which are exactly the additive primaries: cyan + magenta = blue, magenta + yellow = red, cyan + yellow = green. Stack all three and you head toward black (in practice a muddy brown, which is why printers add real black ink, K). Raise the inks below.

Put the two diagrams next to each other and the symmetry is striking. The additive trio meets at white; the subtractive trio meets at black. And each model's secondary colors are the other model's primaries - the two systems are complements. Slide the single intensity control and watch both respond in opposite directions.

"Primary" colors are not magic - they are chosen to span as much color as possible for a given mixing method and a human observer. The additive primaries are red, green, and blue because human vision has three cone types tuned to roughly those regions; RGB lights can stimulate them in nearly independent combinations. The subtractive primaries are cyan, magenta, and yellow because each removes exactly one of those additive primaries - they are the complements.

There is a sneaky third kind. Partitive (optical) mixing does not combine colors physically at all - it places small patches of different colors side by side and lets your eye, which cannot resolve them at a distance, average them into one. It is the principle behind pointillist painting, woven and knitted textiles, mosaic, halftone printing, and the red-green-blue subpixels of the very screen you are reading. Shrink the tiles below and watch a checkerboard dissolve into its average.

Almost every color technology is one of these models - or a clever combination:

Six questions on additive, subtractive, and partitive mixing. Instant feedback, no scores recorded - a wrong answer comes with a short explanation.

Mixing is the foundation; these articles build on each model - the additive screen, the subtractive page, the colorants, and the vision behind the primaries.