Lightness and Brightness and Other Confusions

David Hubel has proposed that the four unique hues or elementary colours of
the opponent system, the psychological primaries, are the only set of colours
that could qualify for the title of primary colours. The evidence in support of
this is quite strong, but there is something in the intangible nature of the
‘psychological primaries’ that will probably make them, quite literally, difficult
to grasp for most practitioners of colour. For those for whom colour is
primarily a creative tool, the idea of the primary, elementary or principal in
colours is so deeply entrenched in our experience of physically mixing
colours, that shifting the concept to something that exists in the mind only is
perhaps too radical. Therefore it makes sense to identify and acknowledge the
various sets of primaries for their own worth, as practical solutions for the
different technological applications or as theories of the psychophysics or the
perceptual psychology of colours. But it is good to remember, that with the
advancement of technology and with our deepening knowledge of perception
and the physiology of colour vision they may still change, both in kind and
number.
Sets of primary colours
The six elementary colour percepts
or the ‘psychological primaries’
red, green, yellow, blue, black, white
The additive primaries (RGB)
violet-blue, warm red, yellow-green
The subtractive primaries
the artist’s primaries: blue, red, yellow
the process primaries: cyan, magenta, yellow
The four process colours (CMYK)
traditionally used in printing
cyan, magenta, yellow, black
Figure 19. Various sets of primary colours.
Vividness of colour
There is one family of colour terms in colour science whose members are
particularly hard to distinguish from each other: saturation, colourfulness,
purity, chroma, and chromaticness. They all refer to the pureness, vividness
or intensity of a colour. They are not, however, synonyms of each other, although
they do all in their own way refer to the same phenomenon. There is
also such a thing as chromaticity, which does not refer to vividness alone, but
also to hue. Most of these terms have several meanings that vary across disciplines
and some have received new meanings along the way. To avoid confusion
I will here refer to the general concept of colour strength as vividness
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