CMY colour printing in practice | lightcolourvision.org

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.

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.