"a dark grey" is correct and usable in written English.
You can use it as a description of any kind of object that is dark grey in color. For example, "The curtains in this room are a dark grey color."
He was wearing a dark grey puffer jacket.
"Kovtun was wearing a silver polyester suit and Lugovoi a dark grey check.
Karadzic, wearing a dark grey suit and tie, stared at the witness over his reading glasses.
Dressed in a dark grey suit, his hair neatly cut, Mladic, 70, seemed unmoved by Mr Groome's assertions.
Dressed in a dark grey suit that sits surprisingly elegantly on his enormous frame, Klitschko speaks slowly and methodically.
OK, I accept that you are going to get wet anyway, but those waves look a lot more ominous under a dark grey sky.
Brandelli has developed this thinking for the spring/summer collection, which has small dots of white and navy running through the cloth of a dark grey suit.
Assange stood in a dark grey suit in the dock as Ouseley began hearing an appeal by British prosecutors acting on behalf of Sweden.
My kitchen is … an airy, light-filled space in Chattanooga, Tennessee, with a white marble counter, light grey floorboards and a dark grey wall.
A black pan, a bowl and a toasting fork sit on a folded white cloth on a dark grey table; the background is a warm brown.
