Skyrider Elf — ruling 1

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 {G}{U} to cast Skyrider Elf for two +1/+1 counters. You can also choose 3 for the value of X, making its mana cost {3}{G}{U}, and pay {W}{U}{B}{R}{G} for five +1/+1 counters.

Skyrider Elf — ruling 2

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.

Skyrider Elf — ruling 3

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.

Skyrider Elf — ruling 4

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 sorcery spells cost {1} more to cast, you could pay {W}{U}{B}{R} to cast Radiant Flames and deal 4 damage to each creature.

Skyrider Elf — ruling 5

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.

Skyrider Elf — ruling 6

If a spell with a converge ability is copied, no mana was spent to cast the copy, so the number of colors of mana spent to cast the spell will be zero. The number of colors spent to cast the original spell is not copied.

Skyrider Elf : règles, fonctionnement et interactions (MTG)

Skyrider Elf is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Flying Converge — This creature enters with a +1/+1 counter on it for each color of mana spent to cast it.

How it works : Skyrider Elf

Original English text — Wizards of the Coast / Scryfall

#1August 25, 2015· Wizards of the Coast
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 to cast Skyrider Elf for two +1/+1 counters. You can also choose 3 for the value of X, making its mana cost , and pay for five +1/+1 counters.
No other card is named in this ruling.
#2August 25, 2015· Wizards of the Coast
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.
No other card is named in this ruling.
#3August 25, 2015· Wizards of the Coast
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.
No other card is named in this ruling.
#4August 25, 2015· Wizards of the Coast
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 sorcery spells cost more to cast, you could pay to cast Radiant Flames and deal 4 damage to each creature.
No other card is named in this ruling.
#5August 25, 2015· Wizards of the Coast
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.
No other card is named in this ruling.
#6August 25, 2015· Wizards of the Coast
If a spell with a converge ability is copied, no mana was spent to cast the copy, so the number of colors of mana spent to cast the spell will be zero. The number of colors spent to cast the original spell is not copied.
No other card is named in this ruling.