I think it's because yellow is supposed to indicate traffic moving in the opposite direction. That's why it's a stronger signal (in theory), but not "correct" for an HOV lane moving in the same direction.
California should have closed the lane once a year for one minute to run a single vehicle down the carpool lane in the wrong direction. Then they could justify that the lane marking was the right color (since in theory, the lane can be reverse-flow at any time, and hey sometimes they even do it).