Most people take for granted that the daylight sky is blue in color simply because it appears blue. They do not per se explore further or ask follow-up questions in that respect. However, what looks one thing may actually be another and so it is only right that one learns to put things in perspective before considering things as facts. It is therefore important to understand things from a clearer and much deeper viewpoint. Let us begin by reviewing first what the word sky means.
It may be difficult for the layman to properly define what the sky is and the difficulty lies in that the sky is only a colored background to contrast with foreground objects that human vision portrays as information in the brain. The sky is not exactly a material object although it is imbued with color, a phenomenon that human vision portrays in the brain through the optic nerve.
In the case of the Earth, which has an atmosphere of air around it, the color of the background called the sky is affected by four factors. One is the predominant source of light from the Sun. Another factor is the size of the nitrogen and oxygen molecules that mainly comprise atmospheric air and in comparison the wavelength of different frequency components of light. The third includes the optical phenomena of absorption, scattering, and diffusion of selective wavelengths of sun light when it impinges on the above molecules in atmospheric air. The fourth factor is surprisingly a specific property of human vision itself.
If you were to stand on the Moon and view the background known as the sky it would not appear blue at all.It would look black and dark the same as the color of the night sky on Earth looks.It is entirely the combination of the above factors that cause you to attribute a certain color to the sky background when viewed in broad day light and in the case of the Earth as appearing imbued with blue color.
The color of an object or background (non-object) therefore depends on not only properties of the object, the predominant source of close by light, and the subtle non-uniformities in the environment. It also depends on the underlying interplay of physics involving the different factors. So, the color of an object or a background should not be taken as its immutable and unchangeable property. The color of an object can therefore change if there is a change in any of the above factors.
Coming back the sky on the Earth simply looks blue in broad direct daylight because the air molecules absorb and then predominantly radiate and scatter indigo and blue colored light (shorter wavelength) much more than the yellow, green, and red color (comparatively longer wavelength) light which pass through unaffected. Human vision is affected predominantly by blue wavelength light more than indigo, so it is the blue color that you see without a trace of indigo.
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