Lightness and Brightness and Other Confusions

We are left with the paint chip. Here we seem to have a paradigmatic case of a
colour quality securely anchored to a physical object. But let’s look a little
deeper. First, suppose that the paint formulation is such that the chip changes
its colour appearance as the illumination changes, perhaps even from one
phase of natural daylight to another. Which of these illumination conditions
reveals its true colour? (Other sorts of materials are even more problematic,
such as dichroic glass or bird feathers whose displayed colours depend upon
viewing angles).
If you think that these are aberrant conditions, put them aside, and consider a
nice stable Munsell or NCS yellow chip. Unless it is large enough to cover
your entire field of view (in which case its perceived colour will quickly fade
into invisibility), it will be seen against some sort of surround. Let that surround
be a neutral grey. But since varying the lightness of the background
will alter the chip’s appearance, which grey will enable us to perceive it correctly?
Or take a more extreme but instructive circumstance. Surround the paint chip
with bright white light, and the yellow is replaced by brown. Alternatively,
take a chip that you would normally describe as brown, keep its illumination
constant, but dim its surroundings, and it will appear yellow (or, depending
on the particular choice of brown sample, orange). It is important to observe
that brown is not just a dim yellow, but rather a blackened yellow; if the surrounding
is kept constant and the illumination is progressively dimmed, the
chip will continue to appear yellow all the way down to the point of invisibility.
Brown is a contrast colour, just like black itself. If a grey scale is viewed
with a lamp equipped with a dimmer, as the illumination increases the grey
scale expands in both directions. By adding light, not only do whites become
whiter, blacks get blacker.
The upshot is that if you dismiss the yellow coloured shadow because it is a
contrast colour, on the same grounds you must dismiss as well the browns
and blacks of everyday life, and these are surely also paradigmatic colours.
Put it another way: our yellow chip is also a brown chip. What colour it is
depends not upon its intrinsic nature but upon the company that it keeps. So
if colours are what we initially thought them to be, none of our samples is
really and truly yellow. If we are to avoid this unpleasant conclusion, we must
refine our notion of colour.
One way to do this is to factor out the observer altogether and content ourselves
with regarding colours as being constituted by spectral power distributions.
This is the world of physical colour. Colour in this sense can be measured
with great precision and needs to be understood very well if one is to
produce and control colorants of all sorts as well as the lighting under which
they are to be seen. Two pigments may look identical and yet have different
spectral power distributions. If each of them is separately mixed with a third
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