basic tips of color/shading.. always think of the light source. when coloring, the shadow color is always the complement (opposite) color of the regular color.
the only thing you need to worry about in coloring is values, i.e the light and darkness relationship, (up and down in photoshop), going left and right doesnt change the value and thus isn't a correct way of shadowing. it has to look good in black & white, and photoshop is a great way of checking that.
i love your soldier dude by the way, i tend to draw futuristic troopers too
Unfortunately, the only place that stock tablets are on the other side of the city, and I have been waiting along time to get one, unfortunately there is no direct route. I'll be getting one when my parent go back there.
BTW, that is impressive for your early projects. Did it only take one pass, or did you build up the layers, or erase previous things?