Monochromatic light refers to light that is of one color. Since it only emits one color, monochromatic light has a single wavelength. Wavelengths are seen as a rainbow of colors when light is passed through a spectrum. Therefore, monochromatic light would emit just one of these colors.
For example, computer monitors emit light containing several wavelengths. However, a monitor that emits only monochromatic light displays just one color on the computer screen. While the color may appear in various shades, light to dark and everything in between, only one color is visible. In most cases, the color is green, red or white.