Each creature with the highest converted mana cost has protection from all colors.
10/1/2007 If there’s a tie for highest converted mana cost, all of those creatures have protection from all colors.
10/1/2007 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.
10/1/2007 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.