WoW! this is beautiful! Congratulation! May I pass you an advice that the great marvel colorist Andres Mossa gave to me when he saw a couple of page of my portfolio? In comics and illustration legibility is key! Make sure to make your characters pop-up more than the environment (unless the environment is more important then the characters in the scene).
Here you've used a realistic atmosphere with characters that blends in the sandstorm, wich is good in a pseudo-realistic rapresentation but it's not really the best in an illustration for a comic. The color of the character are too similar to the color of the background at the point that it make hard destinguish their silhouette, expecialy those in front of the wals. If you have a good flatter layer separate from the rest you can correct this very easily, you don't need to recolour anything, you should just changing a bit tone and satuaration of the characters and it will be perfect.
Don't get me wrong, I love this, expecially the color of the background and the dept you were able to give to it (I'm not sure if I'd be able to do the same as well, in fact I think I would use your image as a reference to improve my own backgrounds XD) It's just a "critic" that I listen every time from professional people and so I though "better reveinving it from a stranger now that from an editor then".
But again, great work! (and forgive my english)

Just awesome! Love it!