The pics of O'brien wearing that yellow penny remind me of how quarterbacks wear special differently colored shirts during practice to remind the defense not to hit them. It's a little reassuring to see that they are practicing with the adidas world cup ball instead of nike balls, so they're used to them.
AP article: At Sunday's final practice, Arena reminded his players numerous times not to take each other down fighting for the ball. He wants them to save that aggression for the games. "They're so competitive," Arena said. "Even in a light day like today, they're banging each other. If they could turn the switch on and off then they wouldn't be the type of competitors they are." The only casualty of the day was midfielder DaMarcus Beasley, who limped off with a slightly twisted ankle after colliding with teammates.
Looks like he's wearing a yellow pinney because he's on the yellow team like Pope and others, not due to 'don't touch him' statue.
My guess is that it's caused by the black rubber pellets in the field turf. They tend to leave marks like that.