Can I run Loyal Cathar in my green-white Commander deck?
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.