Traditional colour theory is derived from the largely Newtonian idea of three primary colours operating in two distinct modes or ’spaces’: The additive or active colour space where colour is dervied from emited or transmitted light, and the subtractive or passive space where we observe colour as it is reflected from an otherwise inert surface.

In the additive space, light is considered to be reducible to three primary ‘channels’ from which the full spectrum of colours can be generated when mixed together. These are the red, green and blues of the ubiquitous RGB acronym. And when these three come together at full and equal volume we have our ‘white’ light.

In the subtractive space, light is also considered reducible to three primary colours but in this case when the siblings converge, the result is black(ish). And notice that each of the primaries in this world are the secondary colours of the other.

So… we could define the pinkish red of the subtractive space (magenta as it’s known in the print trade) as the absence of green; the aqua blue (cyan) as the absence of red; and yellow as the absence of blue. The reverse is also true: if our passive surface (as found in paint, fabric, furniture, animals-that-don’t-glow and people) absorbs all of the yellow, we’re left with blue; if it absorbs the magenta, we observe green; and if cyan is absorbed, we see the result as red.

Traditional colour theory has ruled Britannia forever but it’s not as readily understood in an intuitive psychological sense as ‘modern’ colour theory which is based on two opposing axes (that’s axis-s not axes) each with two endpoints: yello-blue and green-red.