Olo: a new colour that appears only under laser stimulation

Olo is an imaginary colour that can only be seen with specialised equipment that activates the retina's M cone cells without also triggering the L and S cones. Under normal viewing conditions the colour is impossible: the sensitivities of the three cone types overlap across every wavelength of visible light, so no monochromatic stimulus can drive M cones alone. That places olo outside the visible gamut.

How the researchers made it

To work around this, scientists at UC Berkeley mapped a portion of the retina and identified each cone cell as an S, M or L type. They then used lasers to deliver tiny doses of light aimed at the M cones while mostly sparing the other two classes, combining adaptive optics with scanning laser ophthalmoscopy. The results were published on 18 April 2025 in the journal Science Advances, and the colour takes its name from its LMS coordinates (0, 1, 0) spelled out as letters.

What the researchers say

Only the five subjects of the Berkeley experiment have officially seen olo. Ren Ng, a co-author of the study, described it as more saturated than any colour seen in the real world, while participants called it a blue-green of unprecedented saturation. The closest colour inside the sRGB gamut is hexadecimal 00FFCC. Ng and his team are now exploring whether the same technology could ease the symptoms of colour blindness and, potentially, support a form of enhanced vision known as tetrachromacy.

An achievement, but is it a new colour?

The square patch of olo shown to volunteers occupied an angular width of 0.9 degrees, roughly twice the apparent width of the moon seen from Earth. Experts have called the technique a significant technical achievement, but some, including John Barbur of City St George's, University of London, question whether olo is genuinely a new colour, saying the claim is open to argument. Interest has also come from outside science: artists have said they want to create paints inspired by the colour.