You apply cost reduction effects after other cost modifiers, so it can reduce additional costs or alternative costs if they include {W} and/or {B}.

If a spell has hybrid mana symbols in its mana cost, you choose which half you will be paying before determining the total cost. If you choose to pay such a cost with {W} or {B}, Edgewalker can reduce that part of the cost.

If you have more than one of these on the battlefield, the cost reduction is cumulative.