As explained in the discussion of the retinal variable of colour hue, the visual stimulation induced by colour is perhaps the most powerful of the retinal variables with the sole exception of movement.
The power of colour can be its drawback. It is easy to misuse colours and mislead rather than inform. This is why Jacques Bertin (1967) only considered the use of colour as a last resort. In his own words:
“The author has the reputation of being against colour. I am indeed against colour when it masks incompetence; when it allows the superimposition of characteristics to the point of absurdity; when people believe it capable of representing ordered data; for as long as it wastes public funds; by confusing knowledge with publicity; and for as long it is manipulated by the media to convey false and illegible images” (p. 222)
Saturated colours are problematic
Colour saturation is defined as the ‘pureness’ in hue. In more technical terms, a fully saturated hue is achieved when just one wavelength is emitted from the visual spectrum of light and is not contaminated by any other wavelength.
To reduce the saturation of a hue, you can add the hue’s complement.
Saturated red is the worse hue of all. We perceived anything information that is coloured red as being more important than any other colour. To demonstrate this point, Cleveland and McGill (1983) showed the following two maps to random groups of people:
The map of Nevada was equally divided into two parts. In the left-hand side map, half of Nevada is shown with high-saturation red and other half in green colour. On average, 49% of people said that the red area was larger, 22% that green was larger, and 31% that the two areas looked about the same size. When they switched to the low-saturation brighter colors on the right-hand side map, there was no distortion in perception that the areas are indeed about the same.
Evolutionary psychology suggests that hues can also be perceived to having a qualitative meaning. For example, too much exposure to the red hue may cause increased blood pressure hence it may lead to overstatement or overreaction. Colours may also carry cultural meanings, e.g. red in China carries the meaning of celebration and good luck, but in the western world red carries the meaning of warning or danger.
Colour blindness is the limited capacity or total impairment in seeing certain colour hues or differences in colours. There are different forms of colour blindness, with achromatopsia being the most severe form of total colour blindness. Achromatopsia is however rare.
Colour blindness is genetically predisposed, and the genes responsible are on the X chromosome. Women have two X chromosomes which means that a deficient X chromosome can be compensated by the other. Males have only one X chromosome and if it is deficient then it leads to colour blindness.
As explained in the discussion about colour hue, humans have three receptor cones in the retina for receiving short-wavelengths (S-cones, responsible for blue), medium-wavelengths (M-cones, responsible for green), and long-wavelengths (L-cones, responsible for red).
The most common form of colour blindness is red–green, meaning that certain individuals have lost entirely or have limited ability to use their M-cone or L-cone and therefore cannot tell the difference between red and green. For individuals of Northern-European descent, 8% of male and 0.5% of females are red-green colour blind.
Thus, there is a good chance that whoever is reading your graph (e.g. a manager or a customer) is red-green colour blind. This is why we never combine red and green in the same graph.
To understand the extend of the problem, for those who can see the full spectrum of colour hues, here is how red-green colour blind people see colours: