im not sure. You might want to try creating a pattern by hues with value for representing the shape. If all you do is alter the value and keep a consistant palette, it turns out like it is now. For example the green bumpy dude, it has some texture, but the texture itself looks plastic. You could for instance make certain spots like the knees, face and elbos and such have a red tint to suggest that he has some blood under his skin, maybe alter the color of his claws and whatever else is hard bodied to a more suitable color(maybe a more denser/darker color)..
You could change the way the light hits different things.. some things may be fuzzier, some may be shinier, some may refract light..