The eye's ability to pick out colors works to about 500-600 feet for objects the size of our airplanes. But the eye see's the grayscale of the color for much farther. So you pick out two distinctly different colors so you can tell them apart within the range that the color receptors work. And you choose them also to be different shades of gray when farther away. For example, light yellow and navy blue. The yellow will go to light gray and the navy will go to dark gray.

What shade of orange?