Why are stealth aircraft not painted sky blue on the bottom to make them hard to see by looking up from the ground?
At night or with clouds between the plane and ground, the plane won't be visible anyway, so color doesn't matter then. On a clear day, however, wouldn't it be harder to see the plane if it matched the color of the sky?
Because they are rarely tracked by eyeball. Any eyeball guided missile is short range enough to be defeated by altitude. Or if they are low enough, they aren't around long enough for the eye to acquire and track.
Instead, radar. Which does not care about paint color.
But the F-22 and -35 are a pretty much neutral/camo grey. I'd strongly wager that multiple color schemes were tried and tested before coming up with the current paintjobs.
And some are, sort of. The paint for the F-35 65th Aggressor Squadron at Nellis resembles the Russian and Chinese entries. And they are a little bit 'bluish' in places. https://www.overtdefense.com/2022/06/24/u-s-air-force-new-aggressor-paint-scheme-wont-impact-f-35s-stealth/
The sky in most of the world, most of the time, isn't blue. It's various shades of grey. A blue aircraft would stick out like a sore thumb against grey skies. This was found out already during WW2 when your thinking was applied by several nations to paint the bellies of their aircraft. Most of them rather quickly switched to various shades of grey (though possibly economics and availability of blue dyes may have been a factor as well) :)
About the only ones who didn't change to grey during WW2 were the US Navy and Marines, they continued painting their aircraft (dark) blue all over into the Korean war, probably because it might make them harder to spot against the blue water of the sea (which often isn't blue either, but that's another story) as well as because it was tradition by then to paint them blue.
But even they switched to grey by the time of the Vietnam war, when the Air Force was still using woodland camouflage on their aircraft.
The British tried various shades of pink and yellow during both WW2 and Desert Shield/Storm to make detection of low flying aircraft against a desert background harder, but they quickly came back from that (probably in part because repainting aircraft each time they are sent to a different theater is expensive and time consuming).
One other thing to keep in mind is that the bottom of an aircraft is by definition in the "shade" most of the time, that is, the side facing the most away from the sun during most of the daylight hours. As such, looking up at the sky the aircraft will look darker no matter what color it is due to essentially being "lit from behind". Thus, painting a color that matches the sky would essentially be self-defeating because you would see something darker than the sky itself. If you want to minimize the chances of being seen from below the best color might be a matte white, so as to appear as light as possible from underneath, while avoiding reflected flares which would make the aircraft visible from further than usual. This will still not work if the sun is directly or close to directly above (behind) the aircraft as it will still appear almost black due to the extreme contrast in lighting between the light source and the underside of the aircraft.
Of course, as other answers indicate, the aircraft color often makes little difference because of the threat model, e.g. flying too high or too fast for it to make a difference if somebody visually spots it. For slower moving aircraft that may linger it may make sense. Some aircraft such as police helicopters often have white on the bottom, although I doubt it makes much difference.
Coloring of military planes changes, because, and when, typical strategy of use changes.
The first wide used stealth plane, F-117 was colored just black, because it was built only for top secret night missions, and the same considerations chose the color of the B-2.
But other military planes are used in other conditions, or have other considerations, so other schemes were chosen.
Examples:
During WWII, planes were mostly colored green, because they were mostly worried about camouflaging them while on the ground, and it was not considered significant, how far they could been detected in flight;
When, after two world wars, it just so happened that the world pursued for speed, and due to the advent of the rocket/space race, most planes were just polished and covered by a transparent varnish, because it was considered that, their time ending (will lose to rockets and to lasers), and they will be detected by radars, and polished surface considered better for speed;
Then a lot of camouflage schemes were used, each country created its own, some schemes even are different on the top and bottom (Ukrainian planes on top are camouflaged to hide when looking on them with the ground as a background, and on bottom, they are sky blue);
Now, the newest B-21 is shown in light gray, and this fact could tell us that it is considered to be used on typical cloudy day.
I have to say that the newer. Last maybe 5-6 years the flight check aircraft have been painted sky blue top and bottom. As I understand it they were used to check flight procedures in Afghanistan and maybe Iraq I believe. One day when they were newly painted I over heard a pilot ask the controller what they thought of the new paint scheme. The controller responded that it was harder to see. To pilots response what that’s what they were hoping he would say.
