With the team I'm in, I see most people's tab (the list with numbered ID's on the right in the main hub) have a blue color, some are black (that would be me) and the odd yellow one. Now, I know purple means they're on a sortie, it's just the blue/black/yellow thing that's driving me mad. I feel it should be obvious, I just can't see it.  
 
Any help? A manual perhaps Namco? 

I think blue is for workshop

I was under the assumption that blue meant online and black meant offline. I've never noticed yellow though.

From what I've seen, they'll grey out if they're offline, online is still black but brighter, purple means they're on your friends list as well as on your team, blue means they're in the workshop, and orange means they're currently out on a mission. I'm not guaranteeing that's what they are, but just based on what I've seen that's what they mean.

NAT type