The symbol {C} will be used going forward to represent one colorless
mana. The numerical and variable mana symbols ({1}, {2}, and so on,
including {X}) will still be used, but only to represent costs that can
be paid with any type of mana.
example card: To cast Spatial Contortion, you pay two mana: one generic
(that's the {1}) and one colorless (that's the {C}). The generic mana
cost can be paid with any type of mana—that means any color or
colorless. But the {C} is different. That can be paid only with
colorless mana.