Whether green headlights are legal depends on the traffic laws of the state in which the car is being driven. In Connecticut, headlights must either be white or amber. In California, headlights must be white. Some states, including Washington, require that the headlights conform to the federal manufacturing requirements.
The Federal compliance standards for vehicle manufacturing sets out very specific color standards for headlamp colors. It must be within the color boundaries of green, red, blue, white and amber to be legally manufactured. Generally, the requirement states that the headlights must be white, and the taillights must be red. Turn signal lamps should be amber.