How people perceive colour is an interesting issue. The Young-Helmholtz Trichromatic theory of vision proposed that we have red/green/blue receptors, which are then combined to show different colours. It is thought that the red-green receptors are close together, and perhaps this is the root of the issue. Needless to say, the issue is complex but interesting.
According to popular wisdom, it is thought that the red-green chromatic channel developed in order to provide an evolutionary advantage for determining ripe fruits against a background of foilage. Tell that to your children, next time they refuse to eat fruit! However, this ‘ripe fruit’ theory has been difficult to observe in field studies. One group of researchers conducted field studies in black-handed spider monkeys, and found that luminance contrast was just as important in distinguishing fruits. If you’re interested to read more, here is an interesting study that illustrates the complexities of perception, which involves the field study of primates. On the other hand, a separate study showed that trichromatic primates found it easier to determine and select ripe fruits, and you can find more information here.
How does this impact data visualisation? It is possible to produce visualisations that make the most of luminosity in order to encode values, along with the size of the data point, in order to convey the message of the data visualisation. Another issue is that determining colour and luminosity can be a subjective issue, and point size may help to provide additional cues. I envisage it as if it is the detection of fruit in viticulture. Therefore, one winemaker might ascertain that a grape’s optimal point of ripeness is at one point, and another viticulturist might determine that the ripeness point is at another point in time. Similarly, it isn’t always easy to ask experimental subjects to ascertain the amount of ‘greenness’, ‘redness’, or ‘blueness’ of a point. There has been some work in computer vision, aimed at distinguishing the RGB in fruit, which is interesting to read.
It is suggested that about 12% of males are colour-blind, which means that they are restricted from using the red-green channel. If you are interested in reading more about the experience of a colour-blind person, please do read this entertaining blog by Geoffrey Hope-Terry.
To summarise, data visualisations can therefore augment understanding by assisting the perceptual processes involved in luminance and the blue-yellow colours. It is also possible to use the size of the data point to convey the message of the data. In other words, data visualisations should aim not to exclude members of the audience by including lots of red and green together.