Favor of the Mighty

Tribal Enchantment — Giant

Each creature with the highest mana value has protection from all colors.

2007-10-01:
If a creature has X in its mana cost, that X is treated as 0 for the purposes of this effect. It doesn’t matter what the value of X was while the creature was on the stack.
2007-10-01:
If there’s a tie for highest converted mana cost, all of those creatures have protection from all colors.
2007-10-01:
This effect is checked continuously. If a new creature enters the battlefield with the highest converted mana cost, it gains protection from all colors and the previous highest loses it.