You will see that during daytime, a clear sky without clouds is colored blue because the molecules present in the air disperse blue light which is reflected from the sun much more than they spread out red light. If we take a look to the sun during sunset, we can observe that it is colored red and orange combination of colors because the light has been dispersed out and becomes inaccessible to bare eyes.
The white light coming from the sun is blended with all the fascinating colors of the rainbow. This was discovered by Isaac Newton by using a prism to form a separation of various colors creating a spectrum of rays. The colors of light are determined by their inherent wave lengths.
The part of the spectrum which produces red light as can be seen by the naked eye is 720 nm wavelengths. The wavelength towards violet is 380 nm, blended to orange, yellow, green and indigo. The capacity of the human eyes to see colors in the sky is also an important factor to consider.
The retina of the human eye has color receptors of 3 different types which can strongly visualize the colors of red, green and blue wave lengths. This process gives the eyes the ability to have color vision. In 1859, John Tyndall discovered that when light goes thru a clear fluid with tiny particles which are suspended, the smaller wavelengths are spread out stronger than red.
You can observe this by conducting an experiment on a ray of white light directed thru a tank of water containing small amount of milk or soap mixture. On one side, the beam can be visualized by the blue light it deflects, whereas the light which is seen from the end is colored red after passing thru the tank.
The dispersed light can be seen as polarized if you make use of a filter polarized light, just like when the sky shows a color of dark blue using a Polaroid sunglasses. This is commonly known as Tyndall effect but Physicists call it Rayleigh scattering, in honor of the man who conducted deeper study on it a few years after.
Tyndall and Rayleigh theorized that the blue color of the sky is caused by small particles of dust and water vapor in the environment. Much later, scientists concluded that the molecules of oxygen and nitrogen existing in the air are enough proof of light dispersal. In 1911, Einstein quantified the formula for dispersal of light from molecules as confirmed by an experiment. The molecules can disperse light because the electric dipole moments contained in the molecules are induced by the electromagnetic field of the light.
If you look at the sky, the red cones react to the tiny amount of dispersed red light, but lesser to the colors of orange and yellow wave lengths. The green cones react to yellow, green and blue-green wavelengths. If violet and indigo are not present in the spectrum, the color of the sky would be blue with some green polishing.