You can cast Roilmage’s Trick even if your opponents control no creatures. You’ll simply draw a card.

The maximum number of colors of mana you can spend to cast a spell is five. Colorless is not a color. Note that the cost of a spell with converge may limit how many colors of mana you can spend.

 

If there are any alternative or additional costs to cast a spell with a converge ability, the mana spent to pay those costs will count. For example, if an effect makes sorcery spells cost more to cast, you could pay to cast Radiant Flames and deal 4 damage to each creature.

If you cast a spell with converge without spending any mana to cast it (perhaps because an effect allowed you to cast it without paying its mana cost), then the number of colors spent to cast it will be zero.

If a spell with a converge ability is copied, no mana was spent to cast the copy, so the number of colors of mana spent to cast the spell will be zero. The number of colors spent to cast the original spell is not copied.