Did an edit for you. Ill try to explain the best I can.
1: This is your original. I immediately noticed that you had the start of an interesting character but the piece is plagued with common mistakes for beginners but you are already doing pretty well with design so thats a plus! First thing that stood out to me was the color choice. Armor was red, cape was yellow, skull white and so on. You dont want to think so simplistically with colors try to experiment a little Ill talk more about that later on. I also noticed that everything was perfectly outlined in black and that your shading had very little contrast and similar hue.
2:Here I took your original shading and just edited some of the colors and already we are seeing an improvement. If you look at the red colors you have 3 different values. When you chose the shadow and highlight for the red you only pulled up and down on the light value and thats a common mistake. Try increasing the light value while also sliding the hue a bit. In this case I pulled the hue toward yellow since he has a yellow cape and that light would reflect off his armor and pulling colors closer to each other will help your overall color scheme in general. For the shade in the red I pulled the hue to the opposite side of the spectrum more toward blueish purple to give your piece some variation, you don't want it all so red. The piece is still struggling with volume though at this point.
3. In this one I corrected the shading and volumes while trying to leave your original lines intact. The major things I did here was choose a light source which is overhead and to the right a slightly. Its helpful to think of the body as a whole object and not just shade each section seperatly like you did originally. Don't be afraid to move away from your original red color for things that would be hidden in shadow, you can see I did that with the arm,I changed it all to blue with a purple highlight, this will give your piece much more volume and variation. You can also notice this happening in the boots and rib areas. Try not to use pure white or pure greys , pull those hues in the direction of colored objects nearby. You may notice as well that I changed up the line color in certain places depending on the lighting, so where it got really bright near an edge I replaced the black highlight with a purple or blue.
That's a lot of information but I hope it helps you out. If you have any questions I'd be happy to answer them for you. I think you have a good start it just takes some practice, patience and some careful consideration to get better. If I were you I would start practicing with the hue shifting. It's a relatively simple problem to correct and has a drastic effect, just try and get a little more risky with your colors