In fact, the rules show that you can get a Garruk Relentless because it is all the characteristics of both sides:
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200.1. The parts of a card are name, mana cost, illustration, color indicator, type line, expansion
symbol, text box, power and toughness, loyalty, hand modifier, life modifier, illustration credit,
legal text, and collector number. Some cards may have more than one of any or all of these parts.
Some cards may have more than one of any or all of these parts.
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109.3. An object’s characteristics are name, mana cost, color, color indicator, card type, subtype,
supertype, expansion symbol, rules text, abilities, power, toughness, loyalty, hand modifier, and life
modifier. Objects can have some or all of these characteristics. Any other information about an
object isn’t a characteristic. For example, characteristics don’t include whether a permanent is
tapped, a spell’s target, an object’s owner or controller, what an Aura enchants, and so on.
Those things are an object's characteristics.
Therefore, in particular, some cards may have more than one color indicator, which is a characteristic of an object.
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711.2. Each face of a double-faced card has its own set of characteristics.
Indeed, a Garruk Relentless -has- more than one color indicator, one from each set of characteristics you get from each face.
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400.10. An object is outside the game if it isn’t in any of the game’s zones. Outside the game is not a
zone.
Just to establish that none of the rules from 711 apply. (Particularly 711.2a)
With the preceding, I think you can see how one could prove formally that Garruk Relentless not inside the game in fact is Green and Black, and therefore multicolored.