The Earth's sky is blue because the air molecules (largely nitrogen. and oxygen) are much smaller than the wavelength of light. When light. encounters particles much smaller than its wavelength, the scattered. intensity is inversely proportional to the 4'th power of the. wavelength. This is called "Rayleigh scattering," and it means that. half the wavelength is scattered with 2**4 = 16 times more intensity.. That's why the sky appears blue: the blue light is scattered some 16. times more strongly than the red light. Rayleigh scattering is also. the reason why the setting Sun appears red: the blue light has been. scattered away from the direct sunlight.. . Thus, if the atmosphere of another planet is composed of a transparent. gas or gases whose molecules are much smaller than the wavelength of. light, we would, in general, also expect the sky on that planet to. have a blue color.. . If you want another color of the sky, you need bigger particles in the. air. You need something bigger than molecules in the air---dust.. . Dust particles can be many times larger than air molecules but still. small enough to not fall out to the ground. If the dust particles are. much larger than the wavelength of light, the scattered light will be. neutral in color (i.e., white or gray)---this also happens in clouds. here on Earth, which consist of water droplets. If the dust particles. are of approximately the same size as the wavelength of light, the. situation gets complex, and all sorts of interesting scattering. phenomena may happen. This happens here on Earth from time to time,. particularly in desert areas, where the sky may appear white, brown,. or some other color. Dust is also responsible for the pinkish sky on. Mars, as seen in the photographs returned from the Viking landers.. . If the atmosphere contains lots of dust, the direct light from the Sun. or Moon may occasionally get some quite unusual color. Sometimes,. green and blue moons have been reported. These phenomena are quite. rare though---they happen only "once in a blue moon...." :) The dust. responsible for these unusual color phenomena is most often volcanic. in origin. When El Chicon erupted in 1982, this caused unusually. strongly colored sunsets in equatorial areas for more than one year.. The much bigger volcanic explosion at Krakatoa, some 110 years ago,. caused green and blue moons worldwide for a few years.. . One possible exception to the above discussion is if the clouds on the. planet are composed of a strongly colored chemical. This might occur. on Jupiter, where the clouds are thought to contain sulfur, phosphorus,. and/or various organic chemicals.. . It's also worth pointing out that the light of the planet's primary is. quite insignificant. Our eyes are highly adaptable to the dominating. illumination and perceive it as "white," within a quite wide range of. possible colors. During daytime, we perceive the light from the Sun. (6000 K) as white, and at night we perceive the light from our. incandescent lamps (2800 K, like a late, cool M star) as white. Only. if we put these two lights side-by-side, at comparable intensities,. will we perceive a clear color difference.. . If the Sun was a hot star (say of spectral type B), it's likely we. still would perceive its light as "white" and the sky's color as blue.