Why are military aircraft grey?

Beyond a few kilometres an aircraft has no detail left to hide; what gives it away is contrast, the difference in brightness between its surface and the sky behind it. The sky is rarely pure blue: near the horizon and in haze it is a pale, washed-out grey, and clouds are white or grey. A paint matching that average luminance produces the lowest contrast in the largest number of situations. Experiments in the 1970s, notably the counter-shaded schemes developed for the US Air Force's F-15 with the artist Keith Ferris and the Navy's trials on F-4s and F-14s, showed that two or three shades of grey, lighter underneath and darker on top, delayed visual detection more than any green or brown pattern.

Modern paints do more than hide from eyes. Grey coatings are formulated with low infrared reflectance so they do not glow in thermal sights, and on stealth aircraft the top layer sits over radar-absorbent material that dictates the flat, dark grey of an F-22 or F-35. Grey also weathers evenly, hides oil stains and touch-ups, and lets every squadron use the same stock, which a camouflage of several colours does not.

Aircraft that spend their time near the ground still wear green or tan: attack helicopters, some transports and the desert schemes used in Iraq and Afghanistan. Trainers, search-and-rescue and test aircraft are painted in bright colours to be seen, not hidden. And ceremonial or display teams keep vivid liveries because their job is the opposite of camouflage. Grey is therefore not a universal rule but the compromise that suits fighters, which spend most of their lives against the sky.