Rule 118.7e

If a cost is reduced by an amount of mana represented by a hybrid mana symbol, the player paying that cost chooses one half of that symbol at the time the cost reduction is applied (see rule 601.2f). If a colored or colorless half is chosen, the cost is reduced by one mana of that type. If a generic half is chosen, the cost is reduced by an amount of generic mana equal to that half’s number.