There is no green light at the start - it's the lights going out they react to. There's also no minimum time, you can get moving after 1ms - it's legal. In fact

There is no green light at the start - it's the lights going out they react to. There's also no minimum time, you can get moving after 1ms - it's legal. In fact, you can move before the lights go out, there's a tolerance before you're classed as moving.