Tapping a creature won’t reduce colored mana components of harmonize costs.

You don’t choose a target for Glacial Dragonhunt at the time you cast it. Rather, a second “reflexive” ability triggers when you discard a nonland card this way. You choose a target for that ability as it goes on the stack. Each player may respond to this triggered ability as normal.

If a card with harmonize is put into your graveyard during your turn, you can cast it if it’s legal to do so before any other player can take any actions.

You must still follow any timing restrictions and permissions, including those based on the card’s type. For instance, you can cast a sorcery using harmonize only when you could normally cast a sorcery.

“Harmonize [cost]” means “You may cast this card from your graveyard by paying [cost] and tapping up to one untapped creature you control rather than paying the spell’s mana cost,” “If you cast this spell using its harmonize ability, its total cost is reduced by an amount of generic mana equal to the tapped creature’s power,” and “If the harmonize cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack.”

To determine the total cost of a spell, start with the mana cost or alternative cost (such as a harmonize cost) you’re paying, add any cost increases, then apply any cost reductions (such as the cost reduction from tapping a creature). The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.

You can cast a spell using harmonize even if it was somehow put into your graveyard without having been cast.

A spell cast using harmonize will always be exiled afterward, whether it resolves, is countered, or leaves the stack in some other way.