Looks good! Try drawing through the forms so you can see what's on the other side. It's good practice to better understand how the forms sit in 3d space. I attached an example, it's not perfect as I'm also working on this myself. It helps to identify problems when I work on extreme angles for example. Try doing some more exercises from the 3D model and then from real life, eventually some from imagination. If you have trouble identifying the head angle from reference you can go back and try to pose the 3D model as in the reference after a couple attempts. As for when to draw the central line, do what works best for you. I draw it after the circle on side the cranium most of the time.

@Patrick Bosworth here are some more Robo-Beans after your critique, I still have trouble with proportions and identifying the upper box and lower box placement on body sometimes. Checking my work digitally helps with improving though :) thanks

Good work on the sketches! Like @Noe Luis said, focus more on the C, S and I curve lines. As for why the colored sketch looks better there are 2 reasons for that. 1. Colored pencils can only go so dark so mistakes will be less visible on paper compared to darker pencils. 2. The colored sketch you did is actually better and cleaner than what you did in the first uploaded sketch for example. But it is still comparable with the last sketch. I recommend to try and go for longer, cleaner strokes when drawing instead of short, messy ones like in the first sketch. Try practicing capturing the gesture with just 5-6 lines in 30-60 second sketches. And really go for the clean C, S and I lines. If you have trouble with cleaning up your line work try Ghosting your lines. It's where you plan out the line, go through the motion from your start point to the end point without touching the paper, and then execute the mark. You capture gesture well, keep working on the line work too. I hope this helped.