According to Color Matters, "there are more shades of green than that of any other color." Colour Affects points out that green is "in the centre of the spectrum," which, according to Sensational Color, means that on either side it has cooler blue tones and warmer yellow tones.
However, some individuals (mostly men) are colorblind to the colors red and green. This poses a problem worldwide because red and green traffic lights are universal. As Color Matters states, "some European countries have outlined certain traffic light colors so that it is clear which is green and which is red, by the color that has a rectangle around it. Some states in the U.S. have placed diagonal lines through green traffic lights as an aid for the colorblind."