Stop and Go

Shawn first learnt STOP when he saw a photo of a girl (in a magazine) putting her hand out as STOP signal. I taught him the word then. Then we then taught him to STOP at traffic light when the red light is on. So he puts out his hand and say STOP. Recently we taught him to say GO. And he did it easily and he went on ranting Gogogogogogo nonstop. Then we taught him to say Go at green light. When asked green means what? He would say Go... Now when we ask him, red means what? He would say Go. When the taxi stops at red light, he'd shout Gogogo. Now, tell me how to reverse that.