YMC stands for the three colors needed to print a full color id card. The three colors are combined in varying degrees and can print millions of color tones and hues. Yellow, magenta and cyan are the three color panels in each ribbon or cartridge.
Often you will hear the ribbon type YMCKO ribbon talked about. This ribbon prints a full spectrum of colors, but also has a black panel (K) to print black colors and contains a final clear protective overcoat (O) to provide additional protection for the colors printed on the card.