What is a color anyway?
It depends, and that’s the whole problem.
Color isn’t in the light — it’s in your head. Here’s how we exploit that.
[Cold open: the claim that “red” isn’t a property of a wavelength, it’s a property of you looking at it. Set up the whole guide: we’re going to take perception apart, then show why every encoding model since is a clever exploit of how cheap your eye is to fool.]
Three cones, one guess
[The perception foundation, taught deeply. Three cone types (L/M/S), their overlapping spectral response curves, and the central move: three sensitivities collapse a continuous spectrum into a 3-number guess. Your eye is already doing lossy compression before anything reaches your brain.]
Three cone types, each with its own sensitivity, are all that stand between a full spectral power distribution and the three numbers your brain works with.
Drag a wavelength along the visible spectrum; watch the L/M/S cones fire in proportion. Makes “the spectrum becomes three numbers” something you do with your hand instead of read about.
Metamerism: the proof
[The payoff. Two different spectra that produce the same cone response are the same color — identical to you, distinct as physics. This is the proof that color lives in the response, not the light. Land it hard: RGB only works because your eye is cheap to fool.]
Metamerism is the whole trick: it’s why three numbers can stand in for an infinite spectrum.
Second mode of the same widget: drop two different spectra that land on the same combined cone response. Watch them look identical to a synthetic “eye” while being visibly different as spectra. Metamerism, made clickable.
From eye to encoding
[Bridge from biology to model. Why three primaries (because three cones). What actually defines a colorspace: primaries + a white point. Transfer functions are deliberately deferred here — they belong to the ACES guide. Keep this at the door of formal colorimetry; CIE XYZ derivation and chromatic adaptation are a future piece, not this one.]
Gamut: what a space can hold
[What a space can and can’t contain, drawn on the CIE horseshoe. sRGB vs P3 vs Rec.2020 vs ACES as nested triangles. Why wider isn’t free.]
A gamut is fixed by its three primaries and a white point — nothing more.
CIE horseshoe diagram with toggleable triangles for sRGB / P3 / Rec.2020 / ACES. See what each space can hold, and what falls outside it. (A version of this exists in the current color-spaces guide and can be pulled forward.)
Where this goes next
[Short forward link. We’ve established what color is and what a space can hold. The next question — how you move color between spaces without breaking it — is the ACES guide. This guide owns “what color is”; that one owns “how to move it.”]
[Closer line, About-page voice. To be written.]
Glossary
Every term used in this guide, defined once. In the prose, underlined-dotted terms show their short definition on hover or focus, and jump here on click.
- Metamerism
- Two different spectral power distributions that land on the same combined cone response — and therefore look identical — despite being physically different light. The proof that color lives in the response, not the light.
- Cone
- One of three photoreceptor types in the retina, each tuned to long, medium, or short wavelengths. Their overlapping responses collapse a continuous spectrum into three numbers.
- Spectral power distribution
- A curve describing how much energy a light source emits at each wavelength. The full physical description of a color before your eye reduces it to three numbers.
- Gamut
- The triangle of colors a color space can represent, defined by its three primaries. Wider gamuts hold more saturated colors but cost bits and compatibility.
- Color primary
- One of the three reference colors (R, G, B) that define the corners of a color space's gamut triangle.
- White point
- The chromaticity a color space treats as white. Together with the primaries, it defines the space (the transfer function is a separate concern, covered in the ACES guide).