The uniforms of the Imperial Guard are camouflaged in order to protect their wearers by hiding them from sight.

The principle is that what the enemy cannot see he cannot kill. This is not the way of the Adeptus Astartes. A Space Marine’s armour is bright with heraldry that proclaims his devotion to his Chapter and the beloved Emperor of Mankind. Our principle is that what the enemy can see, he will soon learn to fear…

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Let them hide in their fortress. My crew can use the target practice.

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-The Emperor protects.
-And having a loaded bolter never hurt, either.