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.

The colors of mana are white, blue, black, red, and green. Colorless is not a color.

Unless a spell or ability allows you to, you can't choose to pay more mana for a spell with a converge ability just to spend more colors of mana. Likewise, if a spell or ability reduces the amount of mana it costs you to cast a spell with converge, you can't ignore that cost reduction in order to spend more colors of mana.

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 Crystalline Crawler cost {1} more to cast, you could pay {W}{U}{B}{R}{G} to cast it and have it enter the battlefield with five +1/+1 counters.