A fairly scrambled report of a nice piece of work explaining why we see the sky as blue when it is really blue and violet mixed.
http://www.livescience.com/forcesofn..._blue_sky.html

The idea that violet combines with another color to become a blue metamer seems plausible. The solar spectral irradaiance, before it travels through our atmosphere, is heavy in blue and green. It is relatively strong in violet, too.

Violet (around 440nm) is still relatively strong. Violet should scatter between 20% and 75% more than blue, roughly. However, at the greater scatter wavelength (400nm) for violet, our eye is not very receptive . This complicates the matter.

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