Mixing pigments or inks, where combining colours moves toward black — how paint and print work.
Subtractive colour mixing combines materials that each absorb part of the light spectrum and reflect the rest; combining pigments removes more wavelengths, moving the result toward black. Print uses the CMYK model as a practical, if imperfect, subtractive system.
'Mixing red and green paint should give something like screen yellow' is a common misconception — paint mixing is subtractive, so this typically gives a muddy brown instead.
Mixing cyan, magenta and yellow ink at full strength moves toward black, not white.
Explains why mixed paint colours often look duller than expected, and why screen colour and print colour don't always match.
For transparent layers such as inks, dyes and filters, the transmittances multiply wavelength by wavelength, which is the same as saying their absorbances add. For opaque paints, where pigment particles scatter as well as absorb, colour formulation uses the Kubelka–Munk approach: the absorption and scattering coefficients of the ingredients add in proportion to concentration, and reflectance follows from their ratio. Neither calculation can be done on RGB or CMYK numbers; both need spectra.
| Layers | Result | CIELAB L* |
|---|---|---|
| Cyan alone #2dfbfd | #2dfbfd | 90.1 |
| Yellow alone #f6f81a | #f6f81a | 94.8 |
| Cyan over yellow #5ef233 | #5ef233 | 84.9 |
| Magenta over yellow #ed5a43 | #ed5a43 | 57.7 |
| Cyan over magenta #863ef8 | #863ef8 | 46.0 |
| All three #81534b | #81534b | 40.2 |
Cyan over yellow gives the green #5ef233 at L* 84.9, darker than either layer on its own, and all three together fall to L* 40.2, a dark brown rather than a true black. Each layer can only take light away, so a subtractive mixture is never lighter than its lightest ingredient.
How this was calculated: Each ink is a smooth modelled reflectance curve for full-strength sRGB cyan, magenta or yellow; layers are combined by multiplying the curves wavelength by wavelength and viewed under D65. Real inks have less tidy curves, so real overprints differ from these in detail.
A paint-mixing app or a child's lesson predicts mixtures by averaging the RGB values of two paints.
Averaging blue and yellow in RGB gives grey. Real blue and yellow paint give green, because both reflect some green and the mixture absorbs everything else. Only a spectral calculation reproduces that.
A designer expects a printed overprint of two spot colours to look like the two colours multiplied on screen.
Screen blend modes multiply RGB values, which is a rough stand-in for multiplying spectra. The press result depends on the inks' real curves, their transparency and the order they are printed in.