Skull and Rendering Evolution

So, after trying on paper to draw skulls for a couple of days with a ballpoint pen and failing miserably all times except one (the last one, yes), I decided to try drawing them digitally. This is how it went over a couple of days (until I basically had no idea how to improve it, these are my best results, including my first one)

What happened just before I started doing this was that something clicked in my head and I realized that to represent form properly I absolutely must draw light, this means Lights, Shadows and Values (values = shifts in the saturation, lightness or darkness of an immediate surface, the subtle changes), as soon as I realized this, I picked up my not so trusty ballpoint pen and tried drawing a skull by drawing just the darkest shadows, where there would be light or highlight I just let the paper do it's thing and left it be, and there was my first 3D looking skull, it's even slightly angled too! (not front facing like these) using these principles I dove into trying to digitally paint them, and with a new workflow based on rendering the form with shadow and light these are my results.

That green one, the only idea I have for improving it would be drawing it at a higher resolution and further detailing the teeth, then downscaling, but other than that... at this point I ran out of ideas for how to draw the skull from the front any better. Thus now is a time for change; I will proceed to try to draw the skull from other angles, before I move on to bigger and better things
I'm pretty happy with the progress I've made here, but I'm just getting started!  

Update 1: Drew a new (5th) red skull, did a lot of fixing with the anatomy (got the nose ridge right this time, cranial mass is now proportionate and the jaw shape is far more correct. Reason I didn't draw cheekbones is that they're not always clearly visible from this flat frontal angle.) I am now trying to work on my rendering skills, within this skull I started experimenting with having varying levels of saturation (which can be rather clearly seen) and I failed to get a grip on those hard edges. I did not spend as much time on the teeth, I just did the front rows then tried to make the rest fade which eh... kinda worked, but not really. My focus is now getting control over the edges (hard edges in particular) and to do that I'm messing with my brushes, I'm creating my own brush kit as I go through these, I'll release it once I'm happy with the quality level of my renderings which right now I am absolutely not happy about. I will keep updating this file as I draw more skulls, I will no longer be uploading the individual files or processes as I did before (at least not as much) since right now I pretty much draw all these in one go on one layer.

Update 2: Drew a 6th blue skull. cestarian.deviantart.com/art/6…

Update 3: Drew a 7th green skull (First time I draw something and think it looks good I know I messed up the eyebone on the left though, but too late, I'm done!) cestarian.deviantart.com/art/S…

Update 4: Drew an 8th orange skulll... wtf so much better than the others.