To be fair the assignment clearly states “We don’t want to see purple sky’s unless it’s believable.” In this case you have created some purple skies that are borderline believable.
I guess the main thing here for you is to realize that the goal of using your post processing in photography should generally be to make subtle improvements to your images.
At other times if you want to create your own digital artwork than by all means go crazy and have as much fun as you’d like.
But generally with photography subltle changes are good. But there is nothing you can do to take a terrible image and make it a great photograph using post processing.
The standard rule applies, garbage in=garbage out. So always always always get it in camera if you can. And only if you can’t then do you rely on post processing to improve your images with drastic changes.
In this case a huge portion of this image is a black mass and and a sky. Now you have managed to pull out the clouds and make it more interesting but skewing the color so far toward purple it became a bit too unnatural.
Whatever the case the image is more interesting although the color seems a bit unnatural.