Like sleepyhead said, some people use foundation in whatever color. Then for concealor some people prefer to use the opposite shade (Nw to Nc, or Nc to Nw) because it helps neutralize the area, instead of matching it with the entire face. Some people even like to go a shade lighter as well, to give the skin a subtle highlight. Usually for concealing other than under eyes, you should use the same color as your foundation.