text: 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.