Nice sketchbook you have here, I love the Alchemy fighter picture, lots of energy in it. The right arm looks weird because you flex the hand way too much. Do that yourself and you'll see it's quite uncomfortable, and generally only double-jointed people like me can even twist that far >___>. Rotate it 180 degrees and it'll be perfect.
I love the composition in the cave/shaman picture, but the color could be better I think. Why is it green? Is it the light source(in which case, things should be greener in light and reddish/purplish in shadows, and things should turn greener the further away they are)? Or is it the local color of the rocks(it looks like they're brown right now)? Or it could totally be another light source from within the cavern. It looks like the rocks are transitioning from bright yellow to brown to green and then to neutral dark, and it doesn't completely make sense in terms of color theory.
But anyway, unless you intend the colors to be decorative(like let's say, Picasso or Rothko), it's better to explain why the colors are the way they are.
Here are some references I found.