The color key is the list of symbols and corresponding colors that are found on the chart. Most color keys will show the symbol, then the color number found on the skein of floss, then the name of that color.
A value indicating the color to be used for transparent or translucent effects. For example, when using a hardware blitter, all the pixels of a rectangular area are blitted, except the value that was set as the color key; this creates nonrectangular sprites on a surface.
A color key is used to separate one color from an image like how an actor or weatherman is separated from a blue screen. The separated image can then have the color replaced with the visible game world.