FYI, I am back to N4. I should stop posting, all is clear yet -- my face color changes a lot and it seems that a non full cover differs the least over time when I use N4. Now it seems like a really good recommendation from that guy at the mac store.
I have also been playing with some software comparisons. There is no much space between N4 and NC15. By euclidean RGB distance, there is only NW15 in between. A more sophisticated comparison with real pics vs ideal shade colors spat it right out... "take N4". Just to put this here, the order of moving away from the white by euclidean RGB disatance is: N3, NW10, NW13, NC15, NW15, N4, NC20, NW18, C2, C3.5, C3, NW20, N5, ... . I attempted multiple luminosity formulae, but those ones looked less intuitive when showed on screen.
Ask me more if you are curious. This topic bothered me enough that I started working on a software to hlep me. Per above, it is still not good enough to be made public and still has long way to go before that. But if you have similar prob (choosing the right shade) and want to share what you know and learn what I know, we'll start a topic on susans.