Sometimes, the best grey man is "hiding in plain sight". In the suburbs and urban environments, probably the best grey men are guys walking around with fluorescent vests fixing stuff. If they were wearing "boring" clothing while fixing stuff, they'd be more likely to have the cops called on them. I live in the suburbs on a court with little traffic.  Anybody I don't recognize who is walking near my house automatically stands out.  However, if a guy has a fluorescent vest with a "Comcast" logo, I would not think much about him, unless I'm really concentrating
 
Thieves and other life criminal types often disguise themselves as utility or construction workers by wearing hard hats, hi-vis vests etc. 
Any utility or construction worker in my neighborhood who does not appear to have a legit need to be there and seems out of place, certainly gets my attention.
Related links: (many more can be found via a Google search.)  
Burglars in disguise in Mid-City   11 things criminals do when they pretend to be utility workers   Utility impersonators put neighborhood on edge   Home burglars dress up as utility workers   Burglary suspect caught   Suspect poses as worker in job site burglary