Color travels in waves, which are loosely divided into short, medium and long in length. Because colors travel at different wavelengths, some are easier to see than others, but the amount of light is also a factor. In general, however, green is the most visible color from a distance.
Our eyes have three types of photoreceptor cells called cones -- which contain photo-pigments -- that are designed to sense wavelengths. Together, the cones work to communicate to the brain the colors we see. During the day, our eyes are most easily able to pick up green light, followed by yellow and blue. This is one reason traffic lights are green. Red is also used in traffic lights because it stands out against all the green in nature -- even though red is actually the least visible color at a distance.
Rods for Low Light
Along with cones, photoreceptor cells called rods help the eye see during periods of low light. When it's dark, yellow takes over as the most visibly seen color from a distance. This is why many fire trucks are now yellow rather than red, and why many taxis are yellow.