1

This question already has an answer here:

I'm running a green white Commander deck and wanted to know if I could.

marked as duplicate by Toon Krijthe, ConMan, Michael Snook, Joe W, Cascabel Jul 18 '16 at 3:59

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3

Alas, you may not. This is because, per the rules posted on mtgcommander.net,

The back face of a double-faced card is included when determining a card's colour identity.

So, while Loyal Cathar is a white card, a white spell on the stack, and a white creature on the battlefield (or a black creature when it's flipped), its Commander-specific color identity is actually white+black.

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