Human color vision uses three cone types — L, M, and S — sensitive to long, medium, and short wavelengths. Every color we see is a mixture of their activation levels. The gamut of visible color is the set of all activation ratios that natural light can produce. This gamut has a boundary, and we experience it as the most saturated version of each hue — the most vivid red, blue, green. Beyond that boundary, the color space stops. Not because the cones can't respond further, but because no natural light source can drive them there.
The bottleneck is spectral overlap. The M-cone absorption curve extends broadly across the visible spectrum, overlapping substantially with both L and S curves. Any wavelength of light that activates M-cones also activates L-cones, S-cones, or both. There is no wavelength — no single spectral line, no mixture of lines, no physically realizable light stimulus — that activates M-cones alone. The color gamut is bounded not by the biology of the receptors but by the physics of the stimuli.
Roorda, Ng, Doyle, and colleagues at UC Berkeley bypassed the physics (Science Advances, 2025). Using adaptive optics scanning laser ophthalmoscopy — a system that images and targets individual photoreceptors across the retina — they stimulated M-cones exclusively, skipping over adjacent L- and S-cones at the single-cell level. The laser doesn't produce a wavelength that only M-cones absorb. It produces a spot small enough to hit M-cones while missing their neighbors. The selectivity is spatial, not spectral.
Five participants saw a color they named olo — described as a saturated bluish-green beyond anything achievable with natural light. When white light was added to desaturate olo, the resulting mix matched the laser's wavelength, confirming that olo lies outside the normal gamut: it is a color the visual system can encode but that natural stimulation never produces.
The structural observation: the human color gamut is not the range of colors the visual system can represent. It is the range of colors that natural stimuli do produce. The representational capacity exceeds the input space. Three cone types spanning overlapping absorption spectra define a three-dimensional response space, but spectral overlap constrains the accessible region to a subset of that space. The unused region is real — the neural machinery to interpret those activation patterns exists — but it has never been activated in the four hundred million years since trichromatic vision evolved, because no natural stimulus reaches it.
This inverts the usual assumption about sensory limits. The boundary of perception is typically attributed to the sensor: we can't see ultraviolet because our cones don't absorb it; we can't hear ultrasound because our cochlea doesn't resonate at those frequencies. In each case, the sensor defines the limit. For color saturation, the sensor is not the limit. The limit is the entanglement of the stimulus space — the fact that natural light cannot independently address the channels that the sensor provides. The sensor offers a larger palette than the world uses.