Thanks for the encouraging comments, here are some more studies, i`ve found that i hate feet so i`ll be probably drawing a lot of that.
I have something about values and tones in my sketchbook (page 2) from Andrew Loomis, that might help you a little.
About the choice of colours:
To make a picture look good and not make something stick out, colour should be used multiple times on a picture. It doesnt even need to be the exact same colour, reduce the saturation, make it brighter or whatever, just dont make one piece bright green if theres nothing else even close to green.
It can also help when you choose your colour palette beforehand.
Lets say 4 colours that you want to use and you will stick with these (and variations of them).
If you want to go further, mixing colours would be a good way.
The demon up there looks basically good by since the skin is one and the sme colour all over, it feels weird or kinda boring.
Check out old masters paintings and you will see, that there are really fluctuating colours in a field you would say it's basically just one colour.
I hope this helps a bit.
Thanks a lot for the useful advice shockowaffel, you are right, that demon looks boring indeed, i`ll try to limit my palette by choosing some colors beforehand for my next painting and see if i can come up with something decent looking. For now this is how the last one ended up looking, and also a new one.
I really love the last one. So dreamy.