Avacyn, Guardian Angel — ruling 1

Neither activated ability has any effect on damage that’s already been dealt.

Avacyn, Guardian Angel — ruling 2

The possible color choices are white, blue, black, red, and green. You can’t choose artifact, colorless, or any other word.

Avacyn, Guardian Angel — ruling 3

For each activated ability, you choose the color as the ability resolves. However, the color of a source that would deal damage to the permanent or player is checked only when it would deal that damage, not as Avacyn’s ability resolves. For example, if you choose blue, damage a blue creature would deal is prevented, even if that creature wasn’t blue (or wasn’t on the battlefield) when Avacyn’s ability resolved.

Avacyn, Guardian Angel — ruling 4

Whether the target permanent or player is still a legal target is no longer checked after the appropriate ability resolves. For example, if a creature targeted by the first ability gains protection from white after the ability resolves but before it prevents damage, the effect will still prevent damage.

Avacyn, Guardian Angel — ruling 5

Both activated abilities prevent all damage of the chosen color, not just combat damage.

Avacyn, Guardian Angel : règles, fonctionnement et interactions (MTG)

Avacyn, Guardian Angel 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, vigilance : Prevent all damage that would be dealt to another target creature this turn by sources of the color of your choice. : Prevent all damage that would be dealt to target player or planeswalker this turn by sources of the color of your choice.

How it works : Avacyn, Guardian Angel

Original English text — Wizards of the Coast / Scryfall

#1July 18, 2014· Wizards of the Coast
Neither activated ability has any effect on damage that’s already been dealt.
No other card is named in this ruling.
#2July 18, 2014· Wizards of the Coast
The possible color choices are white, blue, black, red, and green. You can’t choose artifact, colorless, or any other word.
No other card is named in this ruling.
#3July 18, 2014· Wizards of the Coast
For each activated ability, you choose the color as the ability resolves. However, the color of a source that would deal damage to the permanent or player is checked only when it would deal that damage, not as Avacyn’s ability resolves. For example, if you choose blue, damage a blue creature would deal is prevented, even if that creature wasn’t blue (or wasn’t on the battlefield) when Avacyn’s ability resolved.
No other card is named in this ruling.
#4July 18, 2014· Wizards of the Coast
Whether the target permanent or player is still a legal target is no longer checked after the appropriate ability resolves. For example, if a creature targeted by the first ability gains protection from white after the ability resolves but before it prevents damage, the effect will still prevent damage.
No other card is named in this ruling.
#5July 18, 2014· Wizards of the Coast
Both activated abilities prevent all damage of the chosen color, not just combat damage.
No other card is named in this ruling.