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.

Widely used subtractive colour models include:
- CMY colour model – a subtractive colour model used to understand the effect of combining cyan, magenta and yellow inks to produce a wide range of colours.
- CMYK colour model – a practical application of the CMY colour model used for printing translucent cyan, magenta, yellow and black inks on highly reflective surfaces.
- RYB colour model – a subtractive colour model used to understand how mixing red, yellow and blue opaque paints can produce other colours.

Subtractive colour with opaque pigments

Subtractive colour models work on the principle that the colour of an opaque object is determined by the wavelengths of light absorbed by its surface and the wavelengths that are reflected.
When someone observes a surface or an opaque object, they perceive the colour based on the wavelengths that are reflected, as all other wavelengths are absorbed.
Mixing opaque pigments together produces a subtractive effect because each pigment absorbs a specific range of wavelengths.
As new opaque pigments are added to the mix, they absorb additional wavelengths, resulting in less light being reflected back towards our eyes.
This reduction in the amount of light that reaches our eyes leads to the perception of a different colour.
Furthermore, as contrasting colours are combined, the resulting mixture appears progressively darker due to the increased absorption of light.

Subtractive colour with translucent pigments

In the case of translucent inks and dyes applied to a material such as paper, an observer perceives a mixture of reflected wavelengths of light plus light that is first transmitted through the ink but then reflected back off the surface of the paper.
The colour an observer sees when looking at translucent inks and dyes can also depend on the properties of the paper including: its surface finish, the angle of the incoming light, and the viewing angle of the observer.

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

About CMY colour printing

CMY printing involves mixtures of three primary colours of dyes or inks – cyan (C), magenta (M) and yellow (Y).
There are two distinct types of CMY digital printing, one involves using solid areas of translucent colour, and the other involves halftoning.
- CMY colour printing using solid areas of translucent colour applies each of the CMY inks to paper in separate layers of solid colour, creating the appearance of different colours and shades by varying the amount of each ink that is applied.
- Halftoning involves dividing each image into a grid of tiny dots and printing each dot in a single colour (typically CMYK) at a fixed size and spacing to create the appearance of different shades and colours.
CMY printing using solid areas of translucent colour can produce less intense or vibrant colours than would be obtained with opaque ink because the translucent inks allow some of the white paper to show through.
Halftoning is the most common method of colour printing used in modern printers, as it allows for high-quality, photo-realistic images to be printed with relatively simple equipment.
In practice, black ink is often added to the CMY inks to improve the depth and clarity of dark areas in the image. This combination of CMYK inks is often used in printing to produce full-colour images with accurate colour reproduction.
Some effects can not be produced using the CMY colour model or CMYK printing.
Screen printing, for example, can use a wide variety of ink types, including spot colours, metallic inks, and special effects inks to achieve results that are unachievable using the standard CMY colour model.
In screen printing, each colour layer is printed separately, and this method often uses spot colours (premixed inks of a specific hue) instead of relying on CMY colour mixing. This allows for more accurate colour matching and vibrant, solid colours.
The use of spot colours can be when only a few colours are needed, as it reduces the number of screens and printing passes required compared to using CMYK colour separation.

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.