Mike,
I purposely painted their backpacks Boltgun Metal, since it's how I painted my Ultramarine backpacks too. When I first got into the game, I wanted a slightly different look to my marines, in comparison to what was depicted in the books. At the time, the easiest part of a Marine to change was the backpack. Even though it's a part of the armor, I painted it differently to accentuate the differences by using another color. So when I started my Imperial Fists, I kind of just went with the same thought process.
Figured that the army's going to have enough yellow in it already, so why not add another neutral color to help it stand out. Thanks for the tip though!
