Hhhmmm....

Color choices look good.

First, I would think about lightening the sky some. It's nearly the same intensity color-wise as your foreground. I would start with that intensity at the top and fade it out the closer you get to the horizon--on a nice sunny day look outside and you'll see what I mean.

Define your clouds a bit more. They don't have to be solid, they don't have to be white, but define them more than what they are now. Don't be afraid to color hold these, either--it's background, color holds in the right shade will push these lines into the background, but do this only if you want.

Think also about creating dimension by "fading" out both the rocky background and the fighters. Do this similar to the sky--lighter, bluer, less saturated colors give the appearance of distance. The further back you want to push an object, the lighter/bluer/less saturated you want to get. Something for you to think about and play around with.

The only other thing I could suggest would be to make sure to add highlights and keep your shadows hard like you have them. You could probably smooth out the shadow lines here or there, like on her right leg (her right, not ours). I probably wouldn't make the inside of the exhaust of the bike bright red--it's not running, so I doubt there should be any light coming from it. Think also about toning the red down on the fighter, at least where it's supposed to be paint and not light.

Not sure what else to say at the moment... It looks like you're just at the basic flat stage right now, ready to really start rendering.