Usually a green screen indicates that the output is set to a different signal. For instance, when the settings are set to HDMI, but the unit is hooked up via component, a green screen can be the resulting picture image.

If the cable/sat box works, the DVD player may be suspect. If you try a different input and it works, then it's likely the DVD player on the fritz. If another input is also black and white and no settings can change that, the unit may require service.