Historical Rules · 1 May 2011
Rule 702.45c
Official source text from the 1 May 2011 Rules release
Generic mana in the sacrificed permanent's mana cost reduces generic mana in the total cost to cast the card with offering. Colored mana in the sacrificed permanent's mana cost reduces mana of the same color in the total cost to cast the card with offering. Colored mana in the sacrificed permanent's mana cost that doesn't match colored mana in the colored mana cost of the card with offering, or is in excess of the card's colored mana cost, reduces that much generic mana in the total cost.