The drop shadow on the clouds are on the wrong side. It needs to follow the isometric perspective of the background for it to look right. Note, you can add a slight drop shadow to the bushes to cary the theme of stage props and add a little depth at the same time. This would help give a continuity to the light source as well.

As for the colors, I don't mind them as they are. I like the "beige feel" as Phlakes called it. However when considering color we all know that it is relative to the other colors respective to it in the space. So by changing the color of just the dirt to a sightly darker shade with less green it can cause a contrast that would make the other colors seem vibrant. Also on that note the Green back drop of this forum can skew the color composition. If we were to mat it on a black background it may not seem so desaturated, so color may not be an issue at all.

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I hope you don't mind but I did a little mock up to explan what I mean. I dropped the image pallat down to 16 colors because you had 20 atm. And I'm not sure what you mean by "using a HUD" so I apologize in advance for my ignorance. I also wanted to use web safe colors but I scraped that Idea when I decided to keep the mock up as close to your color scheme. I don't really feel as though this is an improvement on the peace but I hope it illustrates some things to consider when working on your project.