i LOVE that yellow...
Nc30 and C3 are very similar, and if you are wearing a C3 foundation, you're no where near nc20!!! i would deff take it back to the artist. because you want people to see your beautiful skin, not the beautiful make up.
I understand what you're saying completely...when im tanner, i'm a C3, but right now, I mix NC25 and NC20, or even NC30...but yeah, often times a mac artist will match you with one of these colors, because they are the most common, but it's not the right one exactly.. last summer i went to the beverly hills center mall to the MAC store, and the guy just picked up C3 and swatched it on my cheek, and i don't know if it's because he was in a hurry, or maybe because the lights in that MAC store are dimmed, but i walked outside and my skin was orange....so what i do is i match myself, and then i go outside and look at it. a while ago one of my friends mothers told me that thats how you can tell for sure if you're wearing the right color, take it outside and let the sun decide! hahaha.
if i dont make sense, sorry, im a little sick..so im out of it..hhahaha. i hope it helps.
which brush did you use to apply your pigment?