It is based on six "elementary colors": white, black, yellow, red, blue, and green. Colors are defined by three values, specifying the amount of "blackness", "chromaticity", and a percentage value between two of the colours yellow, red, blue or green. NCS color notations can also be tagged with a letter giving the version of the NCS color standard that was used to specify the color.
Examples of NCS color notation -- the colors of the Swedish flag: