CMY colour model

• The primary colours in the CMY colour model are cyan, magenta and yellow.
• The CMY colour model is a subtractive colour model used with transparent or translucent inks or filters.
• Meanwhile, the CMYK colour model (sometimes called four-colour or process printing) uses the same three primary colours as CMY but uses a fourth component, black ink (K), to increase the density of darker colours and blacks.
• The CMY colour model along with its system of notation enables an exact and reproducible approach to colour printing and other similar applications.
• The CMY colour model is deeply embedded in all contemporary digital printer technologies and underpins industrial standards for the printing industry.

Subtractive colour mixing

• The CMY colour model can be explained by imagining that an observer is presented with a well-illuminated surface such as a highly reflective sheet of white paper.
• In the diagram below a torch is used to illuminate the paper, producing a round pool of light.
• The surface appears white because it is illuminated by white light, so by light containing all the wavelengths of the visible spectrum.
• Cyan, magenta and yellow filters or inks are then placed between the light source and the paper or between the paper and the observer.
• The diagram below shows the effect of placing the filters or patches of ink onto the paper so they partially overlap.

• Where pairs of the primary coloured C, M and Y filters/inks overlap they produce secondary colours and where all three overlap, all wavelengths of light are blocked producing a dark area in the centre.
  • A red filter will transmit red light but absorbs all other colours including green and blue.
  • A green filter will transmit green light but absorbs all other colours including blue and red.
  • A blue filter will transmit blue light but absorbs all other colours including red and green.

• Where two overlapping filters are placed between the light source and the paper or between the paper and the observer the results are as follows:
  • A red filter absorbs both green and blue and a green filter absorbs blue and red, as a result, red, green and blue are all absorbed where they overlap and that area appears black.
  • A green filter absorbs both blue and red and a green filter absorbs blue and red, as a result, red, green and blue are all absorbed where they overlap and that area appears black.
  • A blue filter absorbs both red and green and a red filter absorbs green and blue, as a result, red,  green and blue are all absorbed where they overlap and that area appears black.

• Where all three filters are placed between the light source and the paper or between the paper and the observer the result is that red, green and blue are all absorbed where they overlap and that area appears black.

• Cyan, magenta and yellow filters that correspond with the secondary colours in the RGB colour model but are the primary colours in the CMY colour model behave as follows.
  • A cyan filter absorbs red but transmits green and blue light. Green and blue together appear cyan to the human eye.
  • A magenta filter absorbs green but transmits red and blue light. Red and blue together appear magenta to the human eye.
  • A yellow filter absorbs blue but transmits red and green light. Red and green together appear yellow to the human eye.

• Lastly, where two overlapping CYK filters are placed between the light source and the paper or between the paper and the observer so that they overlap, the results are as follows:
  • A cyan filter transmits green and blue light whilst a magenta filter transmits red and blue. Green and red cancel out producing blue.
  • A magenta filter transmits red and blue light whilst a yellow filter transmits red and green. Blue and green cancel out producing red.
  • A yellow filter transmits red and green light whilst a cyan filter transmits green and blue light.  Red and blue cancel out producing green.

CMYK colour model

• The CMY colour model is helpful in developing an understanding of how combinations of cyan, magenta and yellow primary colours can be used to produce a wide range (gamut) of colours when light is reflected off a surface and wavelengths of light are filtered out by the inks before and reaching the eyes of an observer.
• The CMYK colour model (sometimes called four-colour or process printing) uses the same three primary colours as CMY but uses a fourth component, black ink (K), to increase the density of darker colours and blacks.
• CMYK printing typically relies on:
  • Using white paper with good reflective properties to produce the brightest possible highlights by reflecting the maximum amount of light back towards the observer.
  • Creating highlights by using the minimum amount of coloured ink and printing without black.
  • Producing fully-saturated mid-tones by relying on the brilliance and transparency of printing inks and dyes.
  • Adding black ink when the maximum amounts of cyan, magenta and yellow are insufficient to produce rich black tones in areas of shadow and where black text is required.

Half-tone printing with CMYK

• CMYK is based on the CMY colour model and is the standard model used for colour printing.
  CMYK refers to the four ink plates used in colour printing: cyan, magenta, yellow, and a ‘key’ colour (black).
• Half-tone printing (half-toning) using CMYK involves printing tiny dots of CMY and K in a pattern small enough that humans see solid areas of colour.
• Half-toning allows for a continuous variation in the colour perceived by a viewer through the use of dots, varying either in size or in spacing, thus generating a gradient-like effect.
• Half-tone printing can be used to reproduce black-and-white images by using only black ink and varying sizes or spacing of the dots. This simulates shades of grey, allowing for the representation of continuous tones and gradients in black-and-white images.
• Half-tone black-and-white printing is widely used in newspapers, magazines, and other printed materials.
• The term halftone is sometimes also used to refer specifically to the image that is produced by half-tone printing.