I think I have an idea for that . The Death Star might make an appearance. However, were I to make a stormtrooper or Darth Vadar ball, I would base it off of their helmets.
Really, the Stormtrooper is a simple subject to work with: Black and white, with grey to make some shady suggestions of geometry. He also has some very basic shapes. And being in armor makes him more geometrical rather than organic. Geometrical things can be easier to depict.
Black: Eyes, headband at top, pouty mouth, to black slashes on his cheeks, two vents (one on each side of the face) with black squares representing ventholes. Black chin and two dark dots on either side.
White: the base colour
Grey: Your mid-tone colour, in-between colour, to allow for slight detail without being black or white.
Basically follow your monochrome and you should be fine.
____
/____\
/|> <|\
\ /\ /
\o||o/