In 2025 lasers let people see a colour nobody had seen before
No light in nature can stimulate just one kind of green-sensing cone in your eye. In April 2025 researchers did it anyway, steering a laser onto individual cells. Volunteers saw a blue-green more saturated than anything they had experienced, and the team named it olo, though not everyone accepts it is new.
Human colour vision rests on three types of cone cell, tuned to short, medium and long wavelengths. Their sensitivities overlap, so the brain works mainly with differences between their signals, pooled into opposing channels such as red against green and blue against yellow. A given spot on the retina can signal one side of a pair or the other, never both. That is why ordinary vision offers no reddish green or yellowish blue.
The medium cones are the awkward case. Any wavelength that reaches them also tickles the short or long cones. Extremes work for the others: light around 800 nanometres excites only long cones, and around 360 nanometres only short ones. So stimulating medium cones alone requires bypassing optics entirely, which is what the 2025 group did by imaging the retina and aiming a laser at chosen cells. The name olo comes from its coordinates, 0, 1, 0, in LMS colour space. Colour scientists routinely use such imaginary points as primaries when defining colour spaces, because the region of real colours is curved and no triangle of real primaries covers it all.
You can glimpse related effects at home. Staring at a strong colour tires some cones, and looking next at a different colour yields a chimerical colour. Fixate on bright yellow, then on black, and a stygian blue appears, as dark as the black yet vividly blue. Stare at green, then white, and a red afterimage can seem brighter than the white paper. Cyan followed by orange can produce a hyperbolic orange purer than any real orange light.
Forbidden colours have a contested history. In 1983 Hewitt Crane and Thomas Piantanida pinned red and green stripes to fixed points on the retina with an eye tracker; some viewers saw a colour they could not name, others called it reddish green. In 2001 Vincent Billock and colleagues, controlling for brightness, reported reddish greens and bluish yellows in several subjects. Hsieh and Tse argued in 2006 that such hues are merely intermediate colours, though they relied on steady gaze rather than retinal stabilisation.
Source: Impossible color
