Gray scale refers to the degree to which the brightness of a pixel changes. The number of colors that can be represented by the LED display depends on the gray level of RGB colors, which is 256 levels of gray in the standard full color display. For the stadium full color system, 256 gray is not enough to be accurate because it can not restores the color accurately.
Gray Level is also called color depth. It refers to the number of different brightness. Red, green and blue have their own gray scale. In full color system, it is generally 256 levels of gray, which can produce 256X256X256=16,777,216 colors. It is called a 24-bit color in a PC and is called an 8-bit system in a LED display system.