In Live Chess, you can choose Black if a) you want to play an unrated game, or b) you make a direct challenge to a specific player.
In a), you must look under the New Game tab for the Rating Options link. Click it, then on the lower drop down menu, select "Unrated: Black." Your posted challenges - from then on, until you change it - will be unrated with the Black pieces.
In b), after you click on a name to challenge that specific member, the pop-up menu will give you the option to pick color under the "I play:" drop down menu.
Best wishes.
I wish to practice deence. How do I change to Black?