The night vision refers to the ability to see the images in low lighting condition. It usually appears green in color due to the phosphor screen that makes the objects green. Mostly, the human eyes are sensitive with green color light than the other types of colors.
The phosphor screen with the inbuilt image intensifier, strike the infrared light over the photocathode. This photocathode in then intensified in the image intensifier tube, to produce more electrons and fall over the phosphor plate to form the black and white lights.
The main reason for the night vision devices to look green when viewing is because the human eye feels convenient to look over the green screen than the other colors for a long duration of time.