The backpack of MG Sandrock is not that hard to paint since it has only one color for it's outer parts (purple). However I painted the thrusters with Tamiya XF-56 metallic grey to have some distinction from other metallic parts. Here are the parts of the backpack after painting.
I then put the decals on its designated places.
Parts of the backpack are now group into three major sections.
Here is the actual look of the backpack after being assembled. It can be seen that not all inner parts are covered with armors.
The lower portions of the right and left section can be move to lock the Sandrock's weapon when placed on it's back.
Right and left portions of the backpack can be moved upward and downward, and clockwise and counterclockwise. Articulations are really great!