The pencils in the bottom left throw the whole thing off, because they become out of focus the farther back into z-space they go. To make them seem correct, you'd have to put a blur on everything behind those pencils (which would make 95% of your picture completely blurred out).
I can see where you're going with the "coloring in" type effect, but I think it would be much more effective if you hardened up the edge of the color a bit. If it were actually being colored in, it wouldn't fade out like that (unless you were using watercolors or something, but if that were the case, the paint also would bleed out beyond the lines of the guy).
My one last critique, is the drop shadow on the hand. In order for the drop shadow to look realistic, you're going to have to paint it in by hand instead of using the drop shadow layer style. Reason being is, there are different levels of height in the hand... for instance, there should be just a tiny bit of shadow underneath the pinky, and it should be very close to the pinky, and a have a pretty sharp edge (because it is resting on the surface). Where as the shadow from the paintbrush should be much further away and less sharp (because it's farther away from the surface).
Overall, I think it looks pretty good. Work on those things a bit, and I think you'll have a pretty solid signature