Hey, man. Good sketchbook so far!
I think you should pay a bit more attention to form when painting. What you're doing when painting is to use color and value to communicate 3D form so pay attention to your approach/method. A lot of people (I used to do this too) just paint without any structure behind it and you just end up pushing pixels around aimlessly. Try and decide what you want to do before doing it. Just setup mini goals like "I want to paint this part of the arm as if it turns in space near the edges" and then you spend the time to make it happen before moving on.
In some of your work you kinda have artist-ADD where you don't finish what you started. I think we all have this to some degree. What you've done is that you scribble something down but never take the time to make it look right before moving on. This results in muddy edges, unclean renderings and so on.
I did a quick paintover to show how you could push your latest painting further. A lot of the information was already there, you just need to get better at defining what you're painting.
The main thing I had an issue with was all the yellow rim light. It was everywhere in such a strong amount that it was all you saw. I pulled back on it and focuses more on defining form. A lot of the ideas and groundwork were already there, it just wasn't implemented very well. If you structure yourself and focus on communicating form in a nice way, I'd bet that your work would look much better :)
This image has a lot of color but it isn't handled very elegantly. The same yellow is forced in where it should change in hue and value. When yellow light hits purple metal it doesn't color everything it touches yellow. The effect is more subtle. Slow down before you paint colored on a surface. If you spend the time to think "what will the color be if I add X light to Y surface with it being Z shiny/matte", it will look way better.
You also don't have a lot of color variation. Don't be afraid of just painting in colors. This was one of the things that held me back for a long time. I just couldn't go in on a normal layer and paint in new colors in an image. Variation in hue and saturation is important. It's like the rhythm/gesture of color and finding a nice balance makes for some great work.
Ruan Jia is a person that uses color masterfully. Check out this image (
Link) in photoshop and just look around using the color picker with the color window open. There's not a lot of saturation going on but there is a lot of rhythm in the hue/saturation so it creates a nice balance.
Anyway, I can see that you're getting better so keep on the good work! :)