{C} can be paid only with one colorless mana.

Devoid means “This object is colorless.”

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.”

Annihilator 1 means “Whenever this creature attacks, defending player sacrifices a permanent.”

If a creature is attacking a planeswalker, the planeswalker’s controller is the defending player.

If a creature attacks a planeswalker and that planeswalker leaves the battlefield, the creature continues to attack. If it’s blocked, it’ll deal combat damage as normal, but if it’s unblocked, it won’t deal any combat damage.

Slivdrazi Monstrosity’s ability affects itself, causing it to become colorless and gain devoid and annihilator 1 while it’s on the battlefield. While not on the battlefield, it’s all colors.