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Its not an unreasonable argument, but since Red is the primary treasure color, that just highlights how much they have done to increase the colors relevance in Commander

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You can choose to play the Standard or Expanded format in Casual Mode to refine your skills before challenging experienced players in Ranked Mode

This means that the Background can have a different color identity from the legendary creature and you use the combined color identity of both the legendary creature card and the legendary Background card to determine what the other 98 cards in your Commander deck (or 58 cards in Commander draft) can be