Rule 607.5a

If an object gains an ability that refers to a choice, but either (a) doesn’t copy that ability’s linked ability or (b) does copy the linked ability but no choice is made for it, then the choice is considered to be “undefined.” If an ability refers to an undefined choice, that part of the ability won’t do anything. Example: Voice of All enters the battlefield and Unstable Shapeshifter copies it. Voice of All reads, in part, “As this creature enters, choose a color.” and “This creature has protection from the chosen color.” Unstable Shapeshifter never had a chance for a color to be chosen for it, because it didn’t enter the battlefield as Voice of All so it doesn’t gain a protection ability. Example: A Vesuvan Doppelganger enters the battlefield as a copy of Voice of All, and the Doppelganger’s controller chooses blue. Later, the Doppelganger copies Quirion Elves, which has the ability, “{T}: Add one mana of the chosen color.” Even though a color was chosen for the Doppelganger, it wasn’t chosen for the ability linked to the mana ability copied from the Elves. If that mana ability of the Doppelganger is activated, it will not produce mana.