I would say Shell or Ivory since MAC NC20 is pretty light..
but the best way to do this would be to go into sephora and get someone to test it on your face.
the associates there would be more than happy to help you out there.
I remember once going to sephora and I spent 1 hour with the associate to apply the right tone on me....in sunlight and under bright light....and he skipped his lunch break to help me!!! they're amazing !