Looks like you are using the dodge and burn tools a little bit. They are great tools but you cant rely on them to do all the work for you in the areas of color and light. If you still want to use them, try it just a little bit to pull out a new color and paint with that color/tone.
What this ends up doing is making all your light sources white. If you want a similar process, use the brush tool, and change the setting of the brush to overlay or color dodge and choose a bright color.
Simplifying a few of your forms will make them easier to read for the viewer. These are quite busy with a lot of detail in the forms. Once again, If you look at masters there are only details in the focal points of the entire piece. Use two tones to start by seperating your lights and darks on the forms, then add your third tone for detail.
After you simplify your forms a little bit focus on your detail. All that detail you have is great, but where do you want your viewers to focus? The focal point will deserve all the attention for details.
In these quick paintovers here I added light sources in the form on rim light, because we lose the edge of the figure in the dark background. Your characters have light sources in them like the first one, and light sources are the only reason for color.
Your concepts are great I really like the second design.
The third one seems to have been lost in your brushes. Stick to a few that can make you master form. Airbrush, pastel or chalk brush , pen/pencil brush, and a chisel. I only use about three or four, because when I experiment I lose the painting sometimes.
On the second one here I believe the shirt having sharp edges and being one tone overall seems flat, here I pushed it back into the background with some lost soft edges and darkening it down. This gives us some depth and doesn't attract our eye as much, which leads us back to our focal point, the gooey cool face!!
I hope that helps. If you have any questions let me know. I try to be as clear as possible for critiques. Keep up the good work and lets see more!