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?
closed as primarily opinion-based by anongoodnurse, David, AliceD♦ Dec 6 '17 at 8:41
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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).