I figured that the survival rate in the world of 40k universe is bad enough, why would the space marines not take every advantage as possible. So natural coloration of the enemy is logical.
These are table top quality, but I'm a purist. If you don't want to paint your Ultramarines blue, I say don't play Ultramarines. Anyway, I hardly think painting your armour the same colour as a Tyranid would deceive one.. more likely to end up with friendly fire casualties.
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