Skyrider Elf

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.

You can choose any value for X. The value chosen for X doesn't directly affect the number of +1/+1 counters Skyrider Elf enters the battlefield with, but it does let you pay more mana and thus spend more colors of mana to cast it. For example, you can choose 0 for the value of X and pay green mana blue mana to cast Skyrider Elf for two +1/+1 counters. You can also choose 3 for the value of X, making its mana cost 3 generic mana green mana blue mana , and pay white mana blue mana black mana red mana green mana for five +1/+1 counters.

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.