Charcoal is the safe bet. A mid to light grey is a bolder, but still appropriate choice. Anything else (but white) is dandyish and devil-may-care, if that's what you're going for. Note that, as with many smaller details in one's ensemble, there can be a fine line between looking devil-may-care and looking like you have no idea what you're doing. To avoid being mistaken for the latter, avoid tones that are just a hair's breadth away from charcoal (black, dark navy, very-dark-but-not-quite-dark-enough grey, dark brown, etc.). Basically, the danger zone contains any color that an unconcerned or unknowing neophyte could conceivably have thrown on in a hurry or in the dark.