104.3h.

If a cost would be 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 409.1f). If a colored half is chosen, the cost is reduced by one mana of that color (or, if the cost can't be reduced by one mana of that color, the cost is reduced by one generic mana). If a colorless half is chosen, the cost is reduced by an amount of generic mana equal to that half's number.