Subtractive colour model | lightcolourvision.org

A subtractive colour model explains how different coloured pigments (such as paints, inks, dyes or powders) mix to produce other colours. This concept applies primarily to opaque objects, which don’t allow light to pass through them. Subtractive colour mixing can also be observed with translucent materials that partially transmit light.

Widely used subtractive colour models include:
- CMY colour model: This model is a theoretical foundation for understanding how cyan, magenta, and yellow inks combine to create a wide range of colours.
- CMYK colour model: This is a practical application of CMY, used in printing. It adds black ink (K) to the CMY combination for better contrast and richer blacks, especially when printing on highly reflective surfaces.
- RYB colour model: This model is a historical approach that uses red, yellow, and blue pigments to teach colour-mixing concepts.

Subtractive colour with opaque pigments

All subtractive colour models rely on the principle that the colour of an opaque object or surface is determined by the wavelengths of light it absorbs and the wavelengths it reflects.
When light hits an object, some wavelengths are absorbed by the material, and the remaining reflected wavelengths are perceived by our eyes as colour.
Mixing opaque pigments creates a subtractive effect because each pigment absorbs a specific range of wavelengths.
As more pigments are combined, they absorb even more wavelengths of light.
This reduces the amount of light reflected to our eyes, resulting in a new colour perception.
Additionally, mixing contrasting colours like red and green (which absorb opposite ends of the light spectrum) leads to a darker result because they absorb a wider range of wavelengths combined.

Subtractive colour with translucent pigments

For translucent inks and dyes applied to a material like paper, the observer perceives a mixture of two things:
- Reflected wavelengths of light: Similar to opaque pigments, translucent materials reflect certain wavelengths of light that determine the perceived colour.
- Transmitted light reflected by the paper: Some light passes through the translucent pigment layer and reflects from the surface beneath, such as the paper. This adds another layer of reflected light to the overall colour perception.
The colour an observer sees ultimately depends on both the properties of the translucent ink or dye and the properties of the underlying material (like paper). Here are some factors that can influence the final colour:
- Surface finish of the paper: A smooth paper surface might cause more light reflection compared to a textured surface.
- The angle of incoming light: The angle at which light hits the translucent material can affect how much light passes through and how much reflects.
- Viewing angle of the observer: Depending on the angle from which you look at the coloured material, the interplay of reflected and transmitted light can cause the colour to appear slightly different.

About the CMY colour model and colour perception

A good starting point for understanding the CMY colour model is trichromatic colour theory.
- Trichromatic colour theory explains the underlying 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 states that three monochromatic light sources, one red, one green, and one blue, when mixed together in different proportions, can stimulate the L, M, and S cones to produce the perception of any colour within the visible spectrum.
All colour models, such as the RGB and CMY models, have their foundations rooted in the trichromatic principles of human vision

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About subtractive colour printing in practice

CMY printing involves three translucent inks corresponding with the primary colours – cyan, magenta and yellow.
The CMY colour model is subtractive in the sense that each primary colour can subtract from the light that reaches an observer’s eyes.
In CMY colour printing, colour is applied to the surface of a medium either as dots or as solid areas of colour.
The CMY colour model doesn’t define the exact hue of the three primary colours, so when experimenting with real inks, the results depend on how they are made.

CMY on a white sheet of paper

Cyan ink is painted onto the paper to create a circular shape.
The paper seen through the cyan ink appears cyan to an observer because:
- The ink has absorbed or transmitted all wavelengths of light except those around 500 nanometres (cyan).
- The wavelengths of light around 500 nanometres reflected off the ink, making it look cyan.
- Some transmitted wavelengths passed straight through the ink, reflected off the paper below, passed back through the ink, and added to the intensity of the colour seen by the observer.

Matching patches of magenta and yellow are now painted onto the paper so that areas of each of the three colours overlap.

As already established, the paper seen through the yellow ink alone appears yellow because it has absorbed all wavelengths of light other than those around 500 nanometres (cyan).
Whilst the paper seen through the magenta ink alone appears magenta because it has absorbed all wavelengths of light other than those around 700 nanometres (red).
And the paper seen through the yellow ink alone appears yellow because it has absorbed all wavelengths of light other than those around 580 nanometres (yellow).

Where cyan and magenta ink overlap, the paper appears blue. This is because the cyan ink absorbs red light and allows blue light to pass through, while the magenta ink absorbs green light.
Where magenta and yellow ink overlap, the paper appears red. This happens because the magenta ink absorbs green light and lets red and blue light pass through, while the yellow ink absorbs blue light, leaving only the red light.
Where yellow and cyan ink overlap, the paper appears green. This occurs because the yellow ink absorbs blue light and allows green and red light to pass through, while the cyan ink absorbs red light, leaving only the green light.
Where all three inks overlap the paper appears dark brown.

Remember that in practice, a fourth ink, black (K), is often added to the CMY model to create the CMYK model, which provides better depth and detail in dark areas and helps save ink.
CMYK is commonly used in printing processes like inkjet and laser printing, as well as offset printing for large-scale projects.