Grey is a fairly neutral, very adaptable, and elegant colour; it can be worn with many colours, incl. grey (of course) and black.  The shades of grey determine that too.  In a professional setting the socks are better not lighter than the rest of one's outfit.  Darker colours are better for work, as they look more professional. are less attention-seeking and cope better with the dust on the street and in the office without becoming unsightly.