- Colour Models With Focus on Print Colour Detail

There are two colour models: additive, and subtractive. Screens use additive colour, where red, green, and blue (RGB) are added to each other to create new colours. Print uses subtractive colour, where cyan, magenta, and yellow (CMY) subtract from white light.

Additive relies on light being emitted from a surface, while subtractive relies on light reflecting off a light surface (like paper), which results in it having a much smaller gamut due to limits on how much light can be reflected. Because it can’t reproduce every colour, spot colours (pre-mixed single-colour inks) such as those from Pantone are commonly used.

Two sets of three overlapping circles. The left has red, green, and blue circles, making yellow where green and red meet, magenta where red and blue meet, cyan where green and blue meet, and white in the centre. The right has cyan, magenta, and yellow circles, making blue where magenta and cyan meet, green where cyan and yellow meet, red where magenta and yellow meet, and black in the centre.
Additive on the left, subtractive on the right.

Note that the combinations of two colours in RGB provide CMY, and vice versa. Looking at subtractive, one can observe that:

  • Cyan absorbs red light (reflects green + blue).
  • Magenta absorbs green light (reflects red + blue).
  • Yellow absorbs blue light (reflects red + green).

Print actually uses CMYK, the K representing ‘Key’. Though a CMY alone can create dark colours, it isn’t optimal. The colour is often more of a dark brown than a full black, and printing with cyan, magenta, and yellow usually leads to less fine detail, leaving text fuzzy. For this reason, when printing black, there are three primary different methods used. Standard black (also called flat black or single-colour black) is just straight black ink, which is good for detail such as for text, but doesn’t quite reach pitch darkness. Rich black uses CMYK, meaning it uses those CMY colours paired with black (or ‘key’) ink to achieve a darker, more saturated black. Due to multiple layers of ink, some detail is lost, though.

Even darker is registration black, which is 100% of each cyan, magenta, yellow, and key for the darkest black possible. This does mean a lot of ink (400%) is used, however, which often leads to ink bleeding and smudging, longer drying, and warping of oversaturated paper stock. It is worth keeping in mind Total Ink Coverage (TIC) / Total Area Coverage (TAC), which is typically capped at ~240% for uncoated and ~300% for coated. For this reason, it is usually reserved for crop marks, trim lines, and such.