The gray color reduces the contrast of the ships with the horizon and reduces the vertical patterns in the ship’s appearance. It is the universal color of combatant and auxiliary surface ships, in contrast to the dark gray or black color of submarines, the bright colors of ceremonial vessels and aircraft, or the white of hospital ships and some Coast Guard cutters.
All this applies to present-day practice. There has been a wide variation in paint schemes in the past. There was all sort of color schemes from the end of the 19th century until WWII. The US fleet in the early 20 century was called ‘The Great White Fleet, so you can guess the color Great White Fleet. Dazzle camouflage was popular during WWI. It made it hard to use range-finding devices but was made obsolete by the invention of radar.
The gray paint is a very effective means to keep a warship from being spotted in a wide variety of situations. Gray in some shade is the best color to blend in with the haze and prevent easy visual spotting and identification. If a warship is at or near the horizon, light gray can make it very difficult to spot, even on the sunniest day – which you get less of at sea than you might think. In foggy or cloudy weather, a ship can be even harder to spot. Even if a ship can be spotted at long ranges visually, a uniform gray may delay a ship from being positively identified – and in an era where many ship designs look alike, that can make a difference.
It is also interesting to note that gray is only universal for the side view. The upper decks vary quite a bit. Russian ships use a green and or red deck scheme. US ships are a darker gray known as deck gray. Some navies use green gray etc.
Why are submarines painted black?
Because black reflects the least amount of light and therefore camouflages it the best when it is surfaced, partially submerged, or near the surface. Generally, most of the submarines are painted full black. Some countries paint their submarines in two colors. The top part of the submarine is painted black and the bottom part of the submarine is painted orange. And also, some of the small submarines are painted green.
During WW2 submarines were painted gray, since they spent most of their time surfaced.
If you look closely, you will see the same pattern on fish, such as tuna, such as the blue top and white bottom because to a predator above them, the blue pattern blends with the blue ocean, but if it is a predator below them, the white pattern blends with the white sky.
Keep that in mind while we examine submarine; the primary opponent of a submarine is either other submarines (in which case both sides rely on sonar anyway), aircraft, or ship from above, so they just paint the whole thing with a dark color on it such as black.
The other important point is that it is black not only because of the paint but also because of the type of panels that are used on it. Anechoic tiles are rubber or synthetic polymer tiles containing thousands of tiny voids, applied to the outer hulls of military ships and submarines, as well as anechoic chambers. Their functions are:
(1) To absorb the sound waves of active sonar, reducing and distorting the return signal, thereby reducing its effective range.
(2) To attenuate the sounds emitted from the vessel, typically its engines, to reduce the range at which it can be detected by passive sonar.
Obviously the panels were made black for stealth purposes as everyone else has pointed out.
On the bottom of ship hulls, the Navies use a special epoxy-based red paint to prevent rust and limit barnacle growth. The color is easier for divers to see and indicates to maintenance crews where the underwater hull ends and the upper part begins. But like all things, over time, its luster fades.
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