Historical Rules · 25 August 2017
Rule 117.7e
Official source text from the 25 August 2017 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 colorless half is chosen, the cost is reduced by an amount of generic mana equal to that half’s number.