Historical Rules · 3 July 2020
Rule 118.7e
Official source text from the 3 July 2020 Rules release
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 half is chosen, the cost is reduced by one mana of that color. If a generic half is chosen, the cost is reduced by an amount of generic mana equal to that half’s number.