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, Tower Above is green even if you spend six white 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, Tower Above has a mana value of 6, even if you spend {G}{G}{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 Tower Above by spending {G}{G}{G}, {2}{G}{G}, {4}{G}, or {6}.

When the creature affected by Tower Above attacks, you can target any creature on the battlefield with the triggered ability. However, if the creature you target can't block the creature affected by Tower Above (for example, because the attacking creature has flying and the other one doesn't, or because both creatures are controlled by the same player), the triggered ability does nothing. In that case, the creature you targeted is free to block whichever creature its controller chooses, or block no creatures at all.