You can think of the grayscale as the lightness dimension of the HSL scheme (or the axis of the HSL cylinder)—with saturation held to zero, and hue therefore meaningless. The grayscale is used by photographers, and it is also useful in many documents where variations in gray can be used in place of costly color printing.
Our eyes can actually distinguish about ten different shades of gray, a phenomenon which has been described most eloquently by the famous photographer Ansel Adams, with his “Zone system” of black-and-white negative exposure and print making.
The only possible confusion with a gray scale—always described in percent—is to distinguish between the brightness scale (black = 0, white = 100) and the complimentary scale (100 minus brightness) that printers and photographers use to describe it.
• Black = 0% brightness, 100% gray.
• White = 100% brightness, 0% gray.
• Gray is most often specified from white = 0, thus 10% grey = 90% brightness.