Colour is not a property of objects. It is what happens when light with a particular mix of wavelengths reaches an eye with three types of receptor, and a brain interprets the result against everything else in view. Almost every confusing thing about colour — why paint changes between rooms, why two greys clash, why a screen colour cannot be printed — follows from that one chain.
This section explains the chain, and takes seriously the questions that are usually answered badly. “Is white a colour?” and “is black a colour?” have different answers in physics, in pigment and in ordinary speech, and stating one as the answer is what makes most explanations wrong rather than merely incomplete. Where the evidence is uncertain or contested, these pages say so.
What colour actually is, and why the answer is a chain that runs from light through the eye to the brain rather than a property sitting in an object.
Which colours a single wavelength can produce, which cannot be produced that way at all, and why the rainbow has fewer bands than you were taught.
How three types of receptor and a great deal of neural processing turn a spectrum into the colour you experience.
How colour becomes numbers, why there are so many ways of doing it, and what those numbers can and cannot tell you.
How common or rare a colour is — and why that question needs a scope before it means anything.
The science underpins the practical sections rather than sitting apart from them. Why a paint colour shifts between rooms is the perception chain in action; why a blue feather is blue is structural colour; why a screen green cannot be printed is a gamut limit.