Okay so, ignoring the colours for now, the first thing that pops up is the fact that the shadows are not exactly round. If these are spheres, the shadows should rond off, always (I think always, but just stick with always for now).The best one in my opinion is the purple one with the light coming from the top-right, but the reflected lighting on the bottom is too bright and it shouldn't contrast that harshly with the darkest area.
Try looking at references of shaded spheres.
Also, what is it exactly that you are trying to learn from doing this, the goal is not really clear. Try looking at works from other artists you admire and try to recognise the techniques they use.