503.7.

Because any choices that have been made for a permanent aren't copied, sometimes a copy card will gain an ability that refers to a choice that was never made. In that case, the choice is considered to be "undefined." If an ability refers to an undefined choice, that part of the ability has no effect.

Example: Voice of All comes into play and Unstable Shapeshifter copies it. Voice of All reads, in part, "As Voice of All comes into play, choose a color." and "Voice of All has protection from the chosen color." Unstable Shapeshifter never got a chance to choose a color, because it didn't come into play as a Voice of All card, so the Shapeshifter's protection ability doesn't protect it from anything at all.