Quoting the NYT article @gamepwn mentioned for ease and search results:

She explained: ''They would call you out of the basement, and you'd get a bath and you'd get a dress, and if your dress was yellow you were probably going to Disneyland.'' She said they used color coding to make transactions safer for the traffickers and the clients. ''At Disneyland there would be people doing drop-offs and pickups for kids. It's a big open area full of kids, and nobody pays attention to nobody. They would kind of quietly say, 'Go over to that person,' and you would just slip your hand into theirs and say, 'I was looking for you, Daddy.' Then that person would move off with one or two or three of us.''