11.1 A little colour theory
There have been many attempts to come up with colours spaces that are more perceptually uniform. We’ll use a modern attempt called the HCL colour space, which has three components of hue, chroma and luminance:
-Hue ranges from 0 to 360 (an angle) and gives the “colour” of the colour (blue, red, orange, etc).
-Chroma is the “purity” of a colour, ranging from 0 (grey) to a maximum that varies with luminance.
-Luminance is the lightness of the colour, ranging from 0 (black) to 1 (white).
An additional complication is that many people (~10% of men) do not possess the normal complement of colour receptors and so can distinguish fewer colours than usual. In brief, it’s best to avoid red-green contrasts, and to check your plots with systems that simulate colour blindness. Visicheck (https://www.vischeck.com/vischeck/) is one online solution. Another alternative is the dichromat package34 which provides tools for simulating colour blindness, and a set of colour schemes known to work well for colour-blind people. You can also help people with colour blindness in the same way that you can help people with black-and-white printers: by providing redundant mappings to other aesthetics like size, line type or shape.