i had the same problem, trying to figure out which shade to get (c4 or c40)
i wear nc40-42 in studio fix fluid and c40 for powder. I went through a hassle to find a match i was happy with. I tried the nc's first (nc40, nc42) and both were too weird looking on me. One was too light and the other was too dark (looked orange on me)
i saw the c shades and gave those a try. C4 is more yellow than c40. c40 blended on my skin better than c4, without making me look too light. the MA at mac said that i had olive undertones and that c40 had more olive than yellow.