List of edits: -Switched the values in the sky (lighter closer to the horizon) -squashed the horizon down to push it back -adjusted values and color saturation to integrate characters into the scene, and to emphasize the setting/mood -expanded canvas on top and bottom slightly
It might be too dark (and the brights are far too bright for a night scene), but I wanted to exaggerate a little to make the edits more obvious.
I didn't want to depart too much from unitoone's style, so I refrained from using brushes and limited my tools. The only brushes I used were the eraser and a starfield brush. Tools I did use: lasso, gradient fill, transform, quick masking, layer filters, and blending options.
Skies are darkest towards the top, and lighter closer to the horizon. This is because of the atmosphere.
Skies can be tricky. Instead of trying to invent the colors by picking them from a palette, find an old painting (or photo) of a sky that has the colors/mood you want in your image and color pick (with the eyedropper) from the photo. Don't spend forever doing it, just lay down the basic colors you want (gradient tool is great for nice even "washes"). Then put the photo away so you aren't tempted to waste time by copying it. This will teach you about color. Color picking from old paintings is better than color picking from photos.
But color comes after values (distribution of light and darks), so make sure the values are working first... then jump into the color.
Edit: If my spastic penmanship is hard to read in places, just let me know and I'll translate. >__>
Things that are outside of the scene do not exist.
That is to say, only the artist knows that it exists... and the artist's job to emphasize the effect of the outside-the-scene thing has on the rest of the visible narrative.
The main source of light is not the moon, it's the glowing bits of Star Burst's mane. Perhaps that is not what you intended, but that is how it looks. If the moon was supposed to be the main light source, where is it? Directly above them? East? North, south, west? Could you see it reflecting in the water? What kind of moon is it? Is it full, half, quarter? Maybe a crescent? What does moonlight look like? Is it blue? Or is it silvery? Just things to think about.
Personally, I would add the moon in. Its brightness would balance the darks out, especially if you had it both in the sky and reflecting in the water (a reflecting moon would give you an opportunity to emphasize the calm appearance of the water, and the peaceful nature of the scene.