So the sky is blue because short wavelengths of light coming from the Sun (blue, etc.) are scattered more than long ones (yellow, red, etc.), reflecting the short wavelength light into our eyes instead of it passing through the atmosphere as part of white light. Sunsets are red for the opposite reason … but yeah, why isn’t it violet?
Violet has an even shorter wavelength than blue light. So does indigo, whatever that is. There’s a good logical case for a purple sky, right?
Want to know the answer? Why the sky isn’t violet?
The truth is that the sky is both violet and blue. But the color receptors in our eyes don’t see violet very well, so we get the (incorrect) impression that the sky is just blue. Some birds actually see well into the violet and ultraviolet, so the sky must look trippy as hell to them.