I would never trust MAC makeup artists. If studio fix makes you look yellow, why dont you try studio tech? and studio fix foundtion plus powder is also really good - at times it would make you look cakey, so just use fix spray for that.
I think mac foundations work differently for everyone. What you have to do is figure it out on your own, dont go by their NC and NWs..test different shades on yourself and see which one is the closest match to your skintone.