Normally in a dry sky blue light is scattered by oxygen molecules and you do not see the direct white light unless you look at the sun. In a wet sky white light is scattered by water molecules but not so intense as direct light. In a dry sky with common particulate matter direct light would be scattered beyond the blue wavelength into the yellow longer wavelengths but the sky should become whiter than red and possibly darker if the total scattering increases back into space. I think refractive properties control the redness of a sunset.
Um I think your answer is in your question.
It has to do with the atmosferic scattering of light...I've seen this before too and it was neat as well as eerie...
If the sky is yellow or yellowish, it means that there are many dirt or particulates in the atmosphere.