Prismatic Ending

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 mana value is not a targeting condition. It is checked only as the spell resolves.

You can choose any value for X. Choosing a higher value for X lets you pay more mana and thus spend more colors of mana to cast it. For example, if you choose 0 for the value of X and pay {W} to cast Prismatic Ending, you can exile a nonland permanent with mana value 0 or 1. If you choose 4 for the value of X and pay {W}{U}{B}{R}{G}, you can exile a nonland permanent with mana value 5 or less.