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.