Say hello to chartreuse, or as I affectionately call it, #7FFF00. It’s close to the most visible colour to humans (assuming you have normal trichromatic vision, which isn’t the case in certain forms of colour blindness).

How come? In simple terms, cone cells are little guys in our eyes sensitive to various light wavelengths. When a cone cell that’s sensitive to short wavelengths (aptly called an S-cone) gets hit by such light, it calls hey man, I’m getting struck over here to the brain. It calls loudest in response to a very specific length, but also based on the intensity of the light. That’s why one type of cone doesn’t give us enough information to discriminate colours. We need to compare responses between cones that have different wavelength sensitivities. Shout out to M- and L-cones! While people vary in the exact ratio of cone types that are in their retina, M- and L-cones are always far more common than S-cones.

Here’s the kicker: if you look at a graph of cone responsivity superimposed on the colour spectrum, the point at which the M and L curves intersect describes the wavelength that stimulates them equally–which happens to be a yellow-green colour. Chartreuse is precisely 50% yellow and 50% green, so it should be stimulating your little cones to maximal effect. Voilà!

Note that the above case only holds in well-lit situations. When light levels become lower, rod cells are dominant because they require less light than cone cells to function. With the advent of mesotopic (rod- and cone-based) or scotopic (exclusively rod-based) vision, the eye’s sensitivity shifts to shorter wavelengths, and colour perception becomes poor or near impossible.