Purple is surprisingly hard to get naturally at human survivable temperatures:

Gas
===

[Iodine](https://en.wikipedia.org/wiki/Iodine) is purple gas at temperatures over 185°C (363.7 °F). Unfortunately, Iodine forms compounds with just about everything, many of which are solid or non-purple.

Rayleigh scattering
===================

[Rayleigh scattering](https://en.wikipedia.org/wiki/Purple#Why_distant_mountains_look_blue_or_purple) could cause *distant* objects to look purple, especially if there is a gas in the atmosphere that scatters green light and some blue light<sup>1</sup>. I don't know of any gas that does this at a human survivable temperature.

Other
=====

The native species might perceive blue as purple if they have a red-green colour blindness ([src](http://www.colourblindawareness.org/colour-blindness/types-of-colour-blindness/)) which would make them confuse certain blues and purples. This has other side effects though, mainly being *colour blind*.

Lightning can (briefly) light up the whole sky (clouds in particular) in purple, green, or blue. This might not be what you're looking for.

![sky, with purple clouds][1]

---

<sub>
1. Not sure if there is such a gas, or if I have correctly understood Rayleigh scattering there.
</sub>


  [1]: https://i.sstatic.net/tIg83.jpg