In Western Civilization, at least following the introduction of the automobile and the traffic light, the color green means yes or go, presumably by analogy with green traffic lights. Its opposite in this context is red. To green light a project is to give it the go-ahead.
Green can also mean inexperienced or untrained. This probably comes from green wood, which is flexible and doesn't burn very well because it is young and not yet dried out. Greenhorn is a variation on this, which generally refers to an immigrant, and is generally considered insulting.