The regions of the midbrain that issue subconscious motor commands that affect upper limb position and background muscle tone are the

red nuclei

Explanation:

Red nuclei is the region of the midbrain that issue subconscious motor commands that affect upper limb position and background muscle tone.

The red nucleus is pale pink, it's in the midbrain and it is involved in motor coordination. Its pale pink color is due to presence of iron.

not sure if this what   you'r looking for would be percent composition