Yellowish green, chartreuse, is naturally placed right in the middle of the frequencies of visible light. Human eyes have receptors for green, blue and red colors. Being placed in the middle, chartreuse actuates the most of these receptors to fire, making it distinct and easier to spot. For the same reason, in some metropolises, firetrucks have been modified from red to a yellowish green color to make them more visible and obvious to the eye.
via mindcafe.org
And here I always thought red was the most prominent color.