Hey man, neat sketchbook!
I feel like your pieces are a bit monochrome, try to do a piece with warm lights and cool shadows. I did a quick paintover of your 'Red Forest', as I really like the concept- the contrast is a bit too strong in that one, tune it down by adding some atmosphere (mid value colour. I added blue in the darks and red in the center, and separated the middle/foreground from background). You also have a dual lighting, with both the character having a spotlight and a bright light in the background. Make the piece more interesting by putting the girl into the shade and giving her a nice rimlight around. Again to make the piece more interesting, I'd recommend using blue in the shadows.
Anyway, I hope this helps, keep up the good work and let me know if you have any questions as to why I did something! 
Also, the 'Red Forest' and 'Green Mage' look like they could be part of a series, do something 'blue' next! 
