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Graphite Shadow and Blue Slate are both grayish. The GS is more gray, although a little too light IMO (a little greenish). The BS is darker but you can definitely see the blue. At night, BS can look almost black. You're best bet is to go to a dealership and compare in person. Pictures can be deceiving.
I have a 2011 G37 smokey quartz sedan. It's a great color, as it doesn't show dirt. I wash it about once a month and it really doesn't look dirty in between. It's probably because it's the color of dirt. My previous car was silver and it always looked dirty.