The polish that Cara is wearing is Gris Underground which is an amazing, opaque ash grey shade that would fit right in with the camouflage trend seen on the runways of Michael Kors, Alexander Wang and Christopher Kane.
YSL offers a lovely range of colors. I have previously shown a favourite of mine, Bleu Majorelle which is a great example from the vibrant and dramatic options, and here's an example from the other side of the spectrum with a more toned down, neutral, "city" look. There is definitely a YSL shade to suit everyone and this delicate grey is so pretty and unexpected on your nails.
These polishes have the most amazing wide brush. If I place it in the middle of my nail and gently press down to fan the brush out, it'll cover 99% of my nails on all fingers (except my thumb), so you will actually get as good as full coverage in one clean sweep - it is FANTASTIC! This and the great formula which is a little bit thicker than most, but still very workable - makes it so easy to apply evenly. After one coat you do have super coverage, but since I like my nails very opaque I always have 2 coats + topcoat, which is what I'm wearing in the pic. You will easily get 4 days wear out of this polish and although it is a bit more pricey than some other brands, I look at it as an investment (not that I need an excuse really).
Michael Kors A/W Collection 2013