Shell series 2

Today, I added color to a shell that I drew, and it turned out not so fantastic. I sketched in pencil and then added color with colored pencils. I've been thinking about this for a while, but I can't figure out what medium to use. I thought color pencils would be nice because I have good control, but on the downside, the colors are limited and I can't mix them to get a range of colors. Back when I was in high school, there was this one kid who could make the most incredible and intricate drawings using a selection of colored pencils. All of us were blown away especially because he just decided to try his hand at art in his senior year (no big). Seeing his drawings perhaps has misled me into thinking that I could achieve similar results. Any medium suggestions? I'm currently thinking watercolor with pen outlines or chalk pastels that are more mixable?