If an effect reduces the cost to cast a spell by an amount of generic mana, it applies to a monocolored hybrid spell only if you've chosen a method of paying for it that includes generic mana.

A card with a monocolored hybrid mana symbol in its mana cost is each of the colors that appears in its mana cost, regardless of what mana was spent to cast it. Thus, Reaper King is all colors even if you spend ten colorless mana to cast it.

A card with monocolored hybrid mana symbols in its mana cost has a mana value equal to the highest possible cost it could be cast for. Its mana value never changes. Thus, Reaper King has a mana value of 10, even if you spend {W}{U}{B}{R}{G} to cast it.

If a cost includes more than one monocolored hybrid mana symbol, you can choose a different way to pay for each symbol. For example, you can pay for Reaper King by spending one mana of each color, {2} and one mana each of four different colors, {4} and one mana each of three different colors, {6} and one mana each of two different colors, {8} and one mana of any color, or {10}.