The composition is problematic. You have blurred blades leaves/grass at the left/bottom corner, but they don’t do anything beneficial for your composition. If you want to use blurred foreground elements to create a sense of depth, you should at least use them to frame the main focal point of your composition in an aesthetically pleasing manner.
The rendering of the forms need to be more accurate. You have sudden value shifts that do not describe the forms being rendered adequately, such as as the curvature of her breast–it looks lumpy and lacks smoothness. The lighting also doesn’t make much sense, because you have many contradictions in your lighting such as the position of the highlights. You need to establish your light source(s) and then be consistent about it, instead of guessing and placing highlights and shadows arbitrarily. Think about every single surface and where they face in relation to the light source(s).
The vague background looks too sloppy and unfinished. There’s a huge difference between expressive brushwork and sloppy brushwork. Read this post for more details on this problem: http://forums.cgsociety.org/showpost.php?p=7290573&postcount=4
As for deciding on a color palette, there different ways to do it. The main approaches I use is like this:
-Decide on the overall mood I want to convey, thus establishing the dominant color as well as any complementary colors I would use.
-Figure out how I can logically work that color palette into the scene, such as what type of light sources (including ambient lights or bounced radiosity lights) could produce those main colors casts I need. If the main color casts I need are very artificial looking and do not conform to natural situations, then I need to figure out how this scene can be lit artificially with unnatural light sources (such as man-made lights, magical lights, etc).
-Establish all the neutral local colors for every surface and paint them. Then paint all natural occurring color variations on those local colors, such as discoloration, warm/cool areas on the skin, textures, etc.
-Use a colorize layer to paint in the color casts from light source(s).
-Embellish radiosity/color bleed/bounced lights for extra authenticity and aesthetic appeal.