74 it has to do with generating colorless mana and spending colorless mana for example
{tap}:add {3 colorless} mana to your mana pool
and
{3 colorless} mana:untap this card
had 2 meanings with the colorless mana, at first it was add 3 colorless mana that could only be used as colorless mana and the second one is spend 3 mana of any type. Thus they decided to make a symbol for colorless mana making this card probably read something like this
{tap}:add {C} {C} {C} to your mana pool
and
{3 colorless} mana:untap this card
so.....yea....I get the idea, however I fear for confusion with older cards.
they apparently also changed it slightly that if a card has "{C}: do something stupid" (which will exist) you can spend only, and I really mean only colorless mana on it thus mtg has effectively 6 colors now, white,blue,black,red,green and colorless but don't be afraid as colorless is technically not a color (its the lack of one) mtg has still technically 5 colors
. (and before you say that colorless is not a color as there are no basic lands for it, apparently the new set will have basic lands without a type that generate colorless mana(thus the {C}))