It's frustrating. I usually wear Studio Moisture Tint in medium and Blot Powder in medium. Both are fine, but they aren't covering the redness in my cheeks. I went in to a counter and was sold a mineralized powder that gave me cystic acne. Scratch that. Took it back and they matched me to Studio Fix Powder NC30 (after suggesting NC35). When I went outside with that, I looked like my face was dirty. I went back in and they went down to NC30.
It wasn't really right either and I happened to mention at the counter that MAC foundations just aren't right for me. The artist who often helps me asked what I had and said I was at most a NC25 and to please bring the Studio Fix back. I did, but she was gone by then and the next person tried to match me to C4 (Careblend?). I mentioned that it seemed dark and she said that it was a match for NC35-40. It's ridiculous. I feel like they're matching the darkest part of my cheeks, not my jawline. That artist insisted that my face was darker than my neck and chest (about NC25). I didn't buy the C4; when I went outside I looked like I had a mask on.
I went to the freestanding store and asked the MUA I really like to help. He asked what I was trying to accomplish (natural looking coverage but tone down the redness) and he matched me to N3. I asked about the undertone thing and he said that he felt it would be better at toning down the colour.
Mac Guy mentioned that N colours usually mean NW (clearly not me) so I'm second guessing the N3. I'm just commitment phobic at this point.
Update: I just called MAC back and had a lengthy conversation with a senior artist who said she's been there 14 years. She said that matching lighter tones can be tricky but finds N3 in Face and Body to be fairly neutral, and it's what she would recommend for me in that particular formula (but not in any others). I am happy with the N3 sample I've been using so I think I'll just go with that; it's certainly not as light as inner arms, but it's not noticeably darker than my neck and chest either and everything else has been (although I've obviously blended into my neck).