Proceed with painting of the robot's body and the exposed internal mechanism.
In order to emphasize the sense of distance, let the greenness of the robot's exteriors lower as their positions go background, and try to familiarize them with olive drab.
In order to emphasize the difference from the exteriors, use some kind of reddish brown color for the internal mechanism.

By the way, I designed the skull robot quite rough and didn't care how it looked when it was standing.