Day-Night letters have a face that is one color during the day and another color at night. The ay color could be a dark color like black or bronze and the night color could be white, red or any other LED color. 

The way the system works is the face is covered with a solid color film or print this is what color the face is during the day. For the night color, the film has small holes that allow light from the LEDs to shine through, without being blocked by the day color.