So I bought this studio fix fluid nc40 foundation
and i find it darker than usual
anyone know if i can exchange it at a mac counter ?
I would try to bring it back to the CCO where you bought it, though it might be more difficult as you already bought it at a discounted price. Can you swatch it side by side with your old foundation as a proof? This might help you to get your money back. You definitely can't exchange it at a counter.