Malkior

The explanation could be the height of the atmosphere. A sunset is red because the light has to travel further through the atmosphere, resulting in the lower wavelengths (blue) to be scattered so much that they no longer reach us. So in a thinner atmosphere, we'd see more of the shorter wavelengths, i.e. if Dantooines atmosphere were thinner, it might cause a purple hue in the sky.

Or the most likely explanation: Something purple makes up a significant part of the atmosphere, just like you suggested.

Purple sky news: https://www.theguardian.com/science/shortcuts/2018/mar/19/steve-mystery-purple-aura-rivals-northern-lights-alberta-canada-nasa