Affinity for Elves reduces only the generic mana in the cost to cast Cantankerous Keepers. The colored mana must still be paid.

To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions (such as that of Cantankerous Keepers's first ability). The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast that spell was.