When a monitor display shows only a green image, it is usually because the monitor cable is loose. Display signals are outputted in three primary colors: red, green and blue. A monitor may display only green because the red and blue inputs are not connected. Reconnect the monitor cable, and the full color spectrum should return. If it doesn't, the cable may be damaged.
Monitors use pixels to display images. The three primary colors merge together to create the appearance of every conceivable color, much like mixing the primary colors of paint produces other colors. Viewing a monitor with a magnifying glass reveals the individual pixels that make up each image. Monitor cables transfer the signal of the primary colors from the graphics chip of the computer to the monitor. When a cable is damaged or loose, it usually stops the transfer of one, two or all three of the primary colors. Monitor cables typically have 15 to 24 pins on each connector. Each pin transfers a different signal or color. A damaged or loose cable can result in a predominantly red, blue or green display, depending on which pins are not transferring signals from the graphics chip to the monitor.