Re: quibble - atmosphere of Mars
Rayleigh scattering which gives Earth a blue sky is more efficent for smaller wavelengths through the visible spectrum so in the case of a dust free Mars, it would be blue but not as bright. This is a trade off between the scattering efficency and the visible spectrum of the sun. The mineral aerosols (dust) in the Martian atmosphere complicates this through absorption and Mie scattering.
check these out for the PopSci answer...
http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html
http://humbabe.arc.nasa.gov/mgcm/faq/sky.html