Hey I love your line art there, and the color scheme in the last image.
I saw your stuff on the COW and I thought I might give you some tips if you don't mind. I did a paint-over to illustrate some of my points. If you know all of this just disregard the crap below.
I broke the areas down into letters so it would be easier to describe.
A: This transitional zone watch these areas, and try to add some purple/dark blue into them, try not to use black to deal with the shadow areas.
B: Don't be afraid to overlap the creatures parts, it gives a sense of space. I drew the tail on out to show this. Also add some lighter red to the purple mix to really explore the color spectrum. Remember that when light hits an object it doesn't just move in value, it will also change hues.
C: In exploring hues, add some yellows as hilights to your green bubbles, it will really wake them up and make them round out.
D: In your line drawing the head had a good perspective, I think you lost it a bit when you started painting, I readjusted and painted over it here to try and line it up better.
E: This rounded area shows where shadows will lie and how this arc can determine where your light/shadow boundary will be according to the environment.
F: Adding some deep blue to this mix will really help with the color scheme. It will give a more natural look to the creature.
G: Same here, darker blue and maybe a bit of lighter purple for some bounce light.
H: Add darker green or a contrasting deep dark red, and it will bubble out for you.
Hope this helps, and again if you know all this, just ignore me.