Cones, gamuts, and the color with no wavelength
The Line of Purples
Violet sits at one end of what you can see; red sits at the other. Drag past red and keep going, and you don't fall off the spectrum — you arrive at magenta, a color with no wavelength of its own. Your eye builds it to close a loop the rainbow never actually closes.
Drag the wavelength below. Each cone reports only how much light it caught — never what wavelength caused it. Color exists only in the ratio between the three.
02 — Chromaticity
Why a line becomes a loop
The L cone's job is to catch red light — but opsin chemistry gives it a second, smaller lobe of sensitivity far off at the violet end, an overtone in the molecule the way a plucked string rings with faint harmonics above its main note. So the reddest cone quietly answers when the eye sees deep violet.
That accidental overlap is what bends a straight spectrum into a circle. It gives violet a whisper of "red" it has no business having — and your visual system, always reading ratios, fills in the color that would make the whisper make sense: magenta, stitched across the one gap the spectrum itself never closes.
The shaded band near 410–420 nm is real: it's roughly a fifth as tall as the L cone's main peak, but it's enough to make violet distinguishable from black — and to seed the illusion of a closed color wheel.
The curved edge is every pure, single-wavelength color there is — the spectral locus. The straight edge closing it, the line of purples, corresponds to no wavelength at all: it's what you get mixing the two ends of the spectrum directly, skipping the middle.
03 — Display technology
What a screen can actually show you
A display picks three fixed primaries and mixes them additively. Whatever colors those three points can reach by straight-line mixture is the gamut — a triangle dropped onto the horseshoe above. Everything outside it is a color your eyes can see and your phone cannot.
Phone OLEDs push their red primary out past 620–630 nm — well past where the L cone actually peaks — specifically to separate the L and M response and stretch the triangle wider. No display primary sits in true spectral violet, which is why a laser pointer at 405 nm looks like nothing your screen has ever shown you.
Rec. 2020 reaches much further toward the spectral edge than sRGB, but even it can't touch true monochromatic violet or the line of purples — three straight edges can never fully cover a curve.
04 — Metamerism
Same color, wildly different light
Color isn't a property of a surface — it's what's left after a reflectance spectrum passes through your three cone filters. Two completely different spectra can survive that filtering identically. Below, spectrum A is a smooth, plausible paint reflectance. Spectrum B is mathematically constructed to disagree with it everywhere — and yet be built to feed your S, M, and L cones the exact same numbers.
Push the slider far enough and reflectance runs out of room — it clips at 0 or 1, the trick breaks, and the two swatches finally drift apart. That clipping point is the only reason metamerism has limits at all.
05 — Pigments
Chroma is bandwidth
A pigment's color comes from how narrow a slice of the spectrum it reflects. Cadmium yellow (PY35) reflects a tight, clean band right between the L and M cone peaks and reads as intensely saturated. Earth pigments like raw umber reflect broadly and evenly — useful for neutrality, hopeless for chroma.
Widen the band and watch the swatch desaturate toward grey-yellow — without moving its center wavelength at all. This is also why mixing red, yellow, and blue pigments gives such a muddy green: subtractive mixing widens the surviving band right where your L/M cones are most finely tuned to notice the contamination.
06 — Photosynthesis
Chlorophyll's choice
Sunlight peaks in green-yellow, right where chlorophyll stops absorbing. It catches the blue and red edges and lets the solar peak bounce back at you as green — possibly for photoprotection, possibly historical accident. An absorber actually optimized for the sun's output would look nothing like a leaf.
Purple sulfur and non-sulfur bacteria actually run this second strategy — absorbing green, reflecting magenta. They likely dominated Earth's surface before oxygenic photosynthesis, and survive today pushed into salt flats and sulfur springs by the oxygen chlorophyll itself flooded the atmosphere with, roughly 2.4 billion years ago.