Crono, as stated before, your pencil sketch is very, very nice. Very good lines, good sense of form and loose but clean shading. The color rendition loses the depth that your sketch has. Simply put, jumping from pencil/grayscale to color is quite a challenge, and takes some studying, experimenting, and lots of practice.

The first thing that people assume when using color, is that you use white to lighten the base color, and black to darken it. What this does, is leave you with muddy, gray colors that kill the hue of your base color. The important thing to keep in mind, at first, is that a colors VALUE is the most important thing in defining form. By this I mean, if you convert it to grayscale, where does it fall in the spectrum. If you have the values correct, you can use whatever colors you want, and the form will still be readable.

What I would do, is before you start painting, work out the base color scheme. If it's blue, develop a gradient of 4 blues from darkest blue, to lightest blue. Stick with these 4 gradients, and build the form of the object. Then, once the values are set, and the form works, work on blending colors adding details/texture, etc. Very last step should be adding the extreme highlights and shadows: white, and black. Be careful with these, and use them sparingly.

I had some fun with this image to try and illustrate what I'm saying here. Hope this helps.