The green light really does mean rookie. I read it on one of the team's websites ages ago, where they had a description of each part of the car - and it definitely said that a green light in testing mean a rookie.
It's not a fixed rule, it's just an agreement between teams, so perhaps they needed to put Piquet in a car but didn't have time to change the light over. I don't know. But the green light 100% definitely means rookie. And if I get the time this afternoon, I'll try and back that up with a link or something.