I like the sketch a lot, it has a slick, Steven Silver kind of style to it. Your color rendering of it is just creepy though! There's a lot of great, natural things about the sketch that have been lost - the clean lines have been obscured by over-rendered shadows and highlights. Anytime your image is MAINLY highlights and shadows you know you have a problem - the hair is actually quite good in that regard though! The realistic rendering style is at odds with the distorted, cartoony design. Also, you've changed his posture some - the sketch features a confident, relaxed character - his shoulders are down, head pulled back over his body. The colored version has higher shoulders, and his head is pulled down and pushed forward - combined with the enormous eyes he looks like a paranoid freak! By sticking to a realistic nose and mouth size it puts the eyes further at odds with the rest of the design. Note that your sketch has a nice small nose and a wider mouth, plus a smaller ear. I'd stick with your obviously good instincts and try to work the strengths of your sketch into the finished portrait!