Rule 718.3 · Prototype Cards
As a player casts a prototype card, the player chooses whether they cast the card normally or cast it as a prototyped spell using the prototype keyword ability (see rule 702.160, “Prototype”).
Subrules
While casting a prototyped spell, use only its alternative power, toughness, and mana cost when evaluating those characteristics to see if it can be cast.
Both a prototyped spell and the permanent it becomes have only its alternative set of power, toughness, and mana cost characteristics. If that mana cost includes one or more colored mana symbols, the spell and the permanent it becomes are also that color or colors (see rule 105.2).
If a prototyped spell is copied, the copy is also a prototyped spell. It has the alternative power, toughness, and mana cost characteristics of the spell and not the normal power, toughness, and mana cost characteristics of the card that represents the prototyped spell. Any rule or effect that refers to a prototyped spell refers to the copy as well.
If a permanent that was a prototyped spell is copied, the copy has the alternative power, toughness, and mana cost characteristics of the permanent and not the normal power and toughness characteristics of the card that represents that permanent. Any rule or effect that refers to a permanent that was a prototyped spell refers to the copy as well.