This is cool!
I love the background the most really. I love how detailed it is, and I love how lit you've made the sky.
I also love the colors you've used; though I did have a hard time distinguishing some of the background and foreground. it took me a little while to understand what it was I was looking at with the two characters on either side. But the lineart is less defined than your background; so it ended up being messier, AND more detailed because its closer to your camera. I did figure out the shape on the left, and most of the guy on the right, but he bleeds too much into his background. And he's so complex I can't make out a lot of definable features.
Or perhaps I'm color blind! The world may never know; but I do enjoy your style and execution of color. Just up your contrast a little!