Specifically at MAC-
W in NW:
The W means warm. So this concealer is for warm undertones which are pink toned.
C in NC:
The C means cool. For cool undertones are mainly yellow based tones.
Other than MAC, this image should help you figure out what undertone you have. Any make up artist can help you figure it out, even a friend who knows a lot about makeup.
For cool undertones, you'll tend to have more of a pink/red or blue complexions. Other ways to figure out if you have a cool undertone is to look at your veins form the underside of your arm and they will appear blue, hence why you would show a blue complexion. An interesting way to figure out if you have a cool undertone is by placing a gold and silver piece of jewelry on your skin, which one looks best? Usually the silver one would look great and the gold one too harsh, it may vary, you may look great with both but if you have cool undertones; silver jewelry will look great and the best!
For warm undertones, you'll tend to have green/olive and yellow/golden and even peachy complexions. In this case your veins from the underside of your arm will look greenish tone & it may be hard to even see them! I have warm undertones, which you can already tell because I use NW30 in MAC's studio finish concealer. Gold jewelry looks the best on me! Silver jewelry looks so harsh on my skin & I just can't wear it anymore knowing it just doesn't compliment my own complexion.
If you find yourself in the middle you might just be neutral! Look more into it, there's a lot more websites that can explain thoroughly and more about neutral undertones. I specifically chose to only write about warm and cool undertones.
I hope you found this useful! This is really helpful mostly when looking for certain lipstick shades and like I already mentioned.. jewelry!
P.S. As I go half way into my concealer I will do a review, so far I love it but will have so much to say until then! So stay tuned! I'll keep everyone updated on twitter so follow us @VintageALaVogue