Update: I recently got hold of a sample of NC40 and I don't really like it, it's rather neutral and looks orange on me. I think I've tried so many foundations and foundation shades that have a more truer yellow / olive in them that compliment my skin better, that I find NC40 doesn't have enough now. MAC NC40 was my first foundation that I tried after the constant let downs from pink drugstore foundations so whatever yellow / olive it does have in it was a lot better than the drugstore foundations which is why I thought it was really yellow. But it's actually more on the neutral side.

I thought I'd pick up a sample of the Studio Fix Fluid in NC40. I went home and tried it and was pleasantly surprised at how yellow it was, I looked more golden than usual but I thought it actually looked good! It did make me look a little too yellow but I could deal with that. I used it for the following Eid and mixed it with a lighter foundation to tone down the yellow a bit. It went well, I was commented at how flawless my skin looked and just as I had hoped, it lasted longer than the Studio Sculpt did. I actually didn't find it too cakey or drying on my prone-to-dry combo skin. I obviously moisturised well and I mixed some liquid highlighter into it which I think helped. 

Since I liked the foundation I decided to buy the full size bottle but decided to go down a shade to NC37, thinking that the yellow would go down a notch and be a better match to my skin than the overly yellow NC40. Unfortunately the NC37 wasn't yellow enough and was too peachy undertoned for my yellow skin. I should have stuck to the NC40. I can easily get away with it, since I wear a hijab the face neck difference is concealed, but I prefer to wear foundations that are as close as possible to my skintone. 

Also I noticed that the NC37 oxidises, so it becomes more orange! I'm not sure if the NC40 will do the same, it probably will because I've heard MAC SFF oxidises. I also once mixed in some oil to make it more dewy and moisturising and boy did it turn super orange! You'll see it in the next few swatches!! It's rather difficult taking pictures of swatches and making it resemble the real life swatch. 

I heard that NC37 was a newer shade compared to the rest and also found out that the newer inbetween shades are made more neutral than yellow or pink. I'm not sure if NC35 is more yellow but I do want to try it and see how that compares to NC40. If it isn't I may just have to stick to NC40. I also want to compare NC40 to Nars Stromboli, since Nars Stromboli is the most yellowest liquid foundation I've ever come across and is the closest match to my skintone yet. In comparison to Nars, NC37 is very close to Nars Barcelona. 

Here's what MAC have to say about Studio Fix Fluid: