Generally, lines should be cleaned up before the coloring starts....

but I'd be a hypocrite if I criticized you on that.

Pure white isn't pure white when the sun is setting. The light comes through a whole load of filters in the atmosphere, and it appears orangy to us. Therefore, in sunsets, your ramps should be yellow-orange shifted in general.
Sand casting shadow onto ufo? Check light source: best aa sand into ufo without any of those shadows.
Use those atmospheric shades in the sand. You wouldn't want to be wasting colors now, would you?

Study your AA. There are times where you cross the critical point, and end up blurring the piece, and other times when it just doesn't help with the jaggies.
For the most part, the sand that's disrupted from the UFO crash is the most accurate in the piece. Definitely revise that... err... sand dune on the right side of the piece.
That's all I've got. Good luck in finishing.
