Many effects have you choose a color, card, type or subtype. Examples of cards with abilities like this would include; Adaptive Automaton, Brave the Elements and Meddling Mage. Anytime you are offered a choice like this you must choose something that exists in Magic. When choosing a color, it must be white, blue, black, red or green. When choosing a creature type it must be Advisor, Ally, Angel, Anteater… OK I’m obviously not going to list them all but you get the point.

When you make these choices varies, but it most often happens when a spell resolves or a permanent with a relevant ability enters play. It is the responsibility of both players to make sure that a legal choice is made. If this choice is accidentally skipped over, a judge should be called and he or she should make sure that a legal choice is made at that point.