Due to the carefully calibrated system of layers that Magic uses to determine the interaction of continuous effects, gaining an ability that changes the color of an object has no effect on the color of that object. Despite this, Slivdrazi Monstrosity causes Slivers to become colorless as they gain devoid. Rather than try to figure out how this could work, this card should just be played as though it read “Slivers you control have devoid and annihilator 1 and are colorless.”