The human eye is most sensitive to the color green. However, for some reason red is often selected as the color of distinction or alert. For example, in Renaissance times, painters would outline their figures in a thin red line. In manuscripts, a "red letter" (or "rubric") was used to emphasize an initial capital. Today, we routinely use red for emergency lights or whenever we want to call attention to something. Considering that red is the color our eyes are least sensitive to -- and green the most -- it seems counter-intuitive that we would use red when we want to draw attention to something. Why is this?

  • 2
    $\begingroup$ I think strictly you're mixing two concepts of color. We can talk about a frequency of pure light (or a single photon) in terms of a color under the purview of optics. But optic colors are poor metrics of what we see; instead, we incorporate a theory covering multiple frequencies of light with various intensities (spectra), combined with how they affect our cone sensitivities across an average of humans, under the purview of colorimetry. That in mind, our eyes are most sensitive to optic green, but red lights/paint is colorimetric red. $\endgroup$ – H Walters Dec 2 '17 at 18:55
  • 1
    $\begingroup$ you make a number of unsupported claims throughout your question. Please indicate some prior research to support these claims and to indicate your own effort to answer this question. $\endgroup$ – theforestecologist Dec 2 '17 at 21:12
  • $\begingroup$ Related question: biology.stackexchange.com/questions/24481 $\endgroup$ – vkehayas Dec 2 '17 at 23:16
  • $\begingroup$ Perhaps because (at least in Western Europe and eastern North America, where these conventions originated) most of the world is green for most of the year, while only occasional ripe fruits and flowers are red. Thus red stands out against the green background, while green blends in. $\endgroup$ – jamesqf Dec 3 '17 at 5:06

The reason is more historical than biological.

Our own eyes are more sensitive to green light, and ambient light is mostly green (we are a diurnal being). If you start doing real botany, you will notice that you can distinguish a lot of species just from the shade of green (and for trees, this also from far away).

Red is a difficult colour to produce, and often not very stable over time, and in nature is not so frequent. But it was used because it was also not very common in constructed environments. For example, for the Romans it was a symbol of power (and richness) because it was very expensive to produce red dyes. So red is visible because of the contrast with other colours.

The most visible colour is yellow: the "red" cones are really centred on yellow, and the "green" cones are still very sensitive to yellow. A good yellow appears brighter than any other colours because it stimulates both the medium-wavelength cones and the long-wavelength cones in the human eye. Now emergency services tend to use yellow.

Note: red is not so special (but having a third type of eye cone is very special). Blood is red, but one rarely sees it as red: it quickly turns dark. Fruit? I think blue, green, yellow, orangeish, and dark brown were more common than red (and I recognize a tree by its leaves, from far away, so I'll know what fruit there are).

Traffic light: lights were already very yellowish, and blue, purple, and violets are not that visible, so green, orange, yellow, red, and also white are the most logical colours. I think yellow and white were too similar (especially in cities), because they were the colour of illumination and headlights. Note: in maritime traffic, red, green, yellow, and white are used (not orange).

  • 2
    $\begingroup$ This answer needs citations. Please see our guide to writing good answers in the help section. Thanks! $\endgroup$ – theforestecologist Dec 2 '17 at 21:11
  • $\begingroup$ @theforestecologist: Adding citations is complex: I like the area of colours, but it is not my field, so I know it mainly from secondary sources (and technical point of view, so just as a "user"). If you tell me what one or two important points which requires a reference, I can add, but I give hints to "students", I would really not do their homework. Also considering the most controversial parts of my answer (IMO) are outside biology. $\endgroup$ – Giacomo Catenazzi Dec 4 '17 at 13:42

Not the answer you're looking for? Browse other questions tagged or ask your own question.