Subtractive colour

• Widely used subtractive colour models are:
  • CMY colour model – a subtractive colour model used to develop a general understanding of the effect of combining cyan, magenta and yellow inks to produce wide gamuts of colours.
  • CMYK colour model – a practical application of the CMY colour model used for printing translucent cyan, magenta, yellow and black inks and dyes on highly reflective surfaces.
  • RYB colour model – a subtractive colour model used to understand and manage the effect of mixing red, yellow and blue opaque paints to produce other colours.

Subtractive colours and the human eye

• Good starting point for understanding the CMY colour model is the Trichromatic colour model:
  •  Trichromatic colour theory provides the physiological basis for the subjective experience of colour.
  • Trichromatic colour theory and its precursors have established that there are three types of cone cells (recognised by the initials L, M and S) in the human eye that carry out the initial stage of colour processing that ultimately produces the world of colours we see around us:
    • L = Long (500–700 nm)
    • M = Medium (440 – 670 nm)
    • S = Short (380 – 540 nm)
• Trichromatic colour theory also established that three monochromatic light sources, one red, one green, and one blue when mixed together in different proportions can be used as stimuli the L, M and S cones to produce the perception of any colour within the visible spectrum.
• All colour models including the RGB colour model and CMY colour model have their foundation in this trichromatic underpinnings of human vision.

• The RGB colour model uses the response of the human eye as the basis for establishing rules for mixing red, green and blue primary colours, systems of notation for encoding colours and managing their relationship with one another in any situation where mixtures of red, green and blue primary colours stimulate the eye.
  The most practical colour models are part of everyday life and enable accurate input and output of colour information to TVs, computers, phones and printers.
• The CMY colour model

How CMY subtractive colour mixing works

• CMY subtractive colour mixing involves mixtures of three translucent primary colours – cyan, magenta and yellow.
• The CMY colour model is subtractive in the sense that mixtures of its primary colours subtract from the light that reaches an observer’s eyes and so from the colours they perceive.
• The three colours of ink are either applied to the paper in the form of dots or as solid areas of colour.
• The CMY colour model does not define the exact hue of the three primary colours so when experimenting with real inks the results produced depend upon how they are manufactured.
• To understand how the CMY colour model works in practice imagine a white sheet of paper:
  • A transparent cyan gel filter cut to make a circular shape is now placed on the paper.
  • The paper seen through the gel appears cyan because it has absorbed all wavelengths of light other than those around 500 nanometres (cyan).
  • Matching transparent magenta and yellow gel filters are now placed on the paper so that pairs overlap.
  • The paper seen through the magenta gel appears magenta because it has absorbed all wavelengths of light other than those around 700 nanometres (red) and 450 nanometres (blue).
  • Where the cyan and magenta gels overlap the paper appears blue because the cyan gel has absorbed red but has let blue through.
  • The paper seen through the yellow gel appears yellow because it has absorbed all wavelengths of light other than those around 580 nanometres (yellow).
  • Where the magenta and yellow gels overlap the paper appears red because the magenta gel has again absorbed all wavelengths of light other than those around 700 nanometres (red) and 450 nanometres (blue) but the yellow has blocked the blue.
  • Finally, where the yellow and cyan gels overlap the paper appears green because the yellow has subtracted blue and the cyan gel has subtracted red.
• …
• When exploring the CMY colour model on a computer and generating CMY colour wheels on screen the best results are produced by using a colour picker such as the one shown below.
• Mixing of dyes is used to reproduce a gamut of colours, the resultant colour from this layer is predicted by multiplying (not subtracting)[1] the absorbance profiles of the dyes. This is essentially opposite to the additive colour model, particularly the RGB colour model, that applies to lights whose colour depends directly on the light.
• When the intensities for all the components are the same, the result is a shade of grey, lighter, or darker depending on the intensity.
• When the intensities are different, the result is a colourized hue, more or less saturated depending on the difference of the strongest and weakest of the intensities of the primary colours employed.
• When one of the components has the strongest intensity, the colour is a hue near this primary colour (cyan-ish, magenta-ish, or yellow-ish)
• when two components have the same strongest intensity, then the colour is a hue of a secondary colour (a shade of red, green or blue).
• A secondary colour is formed by the sum of two primary colours of equal intensity:
  • red is magenta+yellow,
  • green is yellow+cyan
  • blue is cyan+magenta.
• Mixing secondary colours will result in the dark versions of their common primary colour:
  • green+blue is dark cyan or teal
  • blue+red is dark magenta or purple
  • red+green is dark yellow or olive.
• Every secondary colour is the complement of one primary colour:
  • red complements cyan,
  • green complements magenta
  • blue complements yellow.
• When a primary and its complementary secondary colour are added together, the resulting colour would be the very dark version of the mixed primary colour:
  • cyan+red results into very dark cyan or dark teal,
  • magenta+green results into very dark magenta or dark purple
  • yellow+blue results into very dark yellow or dark olive
• When all the primary colours are mixed in equal intensities, the result is black.

Absolute colour space