Themes
Say which colour
A swatch is a set of coordinates in a space, under an observer, on a display with a gamut. Every colour drawn here carries all four, because a hex code on its own does not identify a colour at all.
Outside the gamut
Most colours a human can see cannot be shown on this page. The usual response is to clip them silently and print the picture anyway. The response here is to mark them and say how far out they are.
Identical, and asserted
"These two patches are the same colour" is the most repeated and least checkable sentence in perception, because the whole point is that it does not look true. Every instance here is an assertion in code.
Three numbers
An infinite-dimensional spectrum is projected onto three cone responses, and everything colour science can do — and every way it fails — follows from that single collapse.
Matching is not appearance
CIE XYZ predicts when two lights will match under identical viewing conditions. It was never a model of how anything looks, and most of the confusion in applied colour comes from using it as one.
Computed, not quoted
Every swatch begins as a spectral power distribution and is carried through the colour-matching functions while the figure is drawn. None is a hex code recalled from a table.
The instrument is the reader
This is the one subject where the page is displayed on the apparatus under discussion, and the reader's own eye is the measuring device. Several figures here are experiments rather than illustrations.