I'm also having this issue, and it's kinda annoying!
Looks like neither of you have the latest driver for your graphics cards. Try downloading this one and see if it fixes the purple sky;
I was finding I had a few issues on several games so have since upgraded my graphics card...but the catalyst drivers etc I had downloaded for my old ATI card were 10.2 as diercted to by the link and I still had the problem. Anyway, my sky is normal now and I can at last enjoy the game
You mean you had the puple sky with your previous card and 10.2?
May i ask what card it was and what card you have now.
For me started the problem after i did reinstall NAWC with newest network checked.
This solved my problem with not showing mouse pointer but caused this purpe sky.