There can be a whole host of reasons we may see a purple sky - dust, pollution, water droplets, and cloud formations can all influence the colors of the sky. In addition to these influences, wavelengths play a large part in what colors we see in the sky.
Occasionally, pink and purple will appear partially due to "the optical illusion of the pink wavelengths lighting up the base of the cloud, and these pink clouds superimposed on a dark blue sky. The combination of pink and dark blue can make the sky appear a deep purple."
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