The answer to the question why is the sky blue during the day when there are no clouds is the presence of molecules in the atmosphere that serve to scatter away from the sun blue light more than the red light. When we take a look towards the sun in the course of sunset, we see orange red colors owing to the fact that the light that is blue in color has already been scattered away and out of the sight line.
The white light that is produced by the sun is a blend on all the colors on the rainbow. This was shown by Isaac Newton who made use of a prism to distinguish different colors and thus result in a spectrum. The light colors are separated using the differing wave lengths. The part of the spectrum that can be seen ranges from light which is red in color that has a wavelength of approximately 720nm to violet light that has a wavelength of approximately 380nm having indigo, blue, green, yellow and orange in the middle. The three distinguishing kinds of color receptors found in the human eye retina respond with much strength to blue, green and red wavelengths providing us with the color vision.