Historical Rules · 18 June 2010
Rule 116.7c
Official source text from the 18 June 2010 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 601.2e). 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.