Hi there! Great start to your sketchbook! :) Really like the first concept and the environments are pretty cool, too.
As for colors, I'd say you could use some more saturation, especially in that mordor piece I see too much grey. I don't know if you used this reference:
http://www.centives.net/S/wp-content/upl...ordor1.jpg but if you did, you can check with the colorpicker and see how the base of the mountain for example is more saturated than in your version, also the ground and the sides of the tower are a lot more blue than you made them. Also, while the fog around the mountain is greyish in the original, too, it tends towards a reddish grey which generates a better contrast in front of the blueish mountain =)
The same goes for the first portrait, I think you tended towards too undersaturated colors, here, too. The last portrait is indeed more colorful, but even here you might want to try out how it looked if you chose a little more saturation in some parts.
Hope that helped, keep it up!