Prismatic Ending

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.

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.