Yellow icon means that this neighbour has recently sped up or collected buildings in your city (no earlier than 5 days ago). "Yellow" neighbours are displayed first on the list so that you could see who's helping you. If you help a "yellow" neighbour in return, the icon will turn blue.
Blue icon means that this neighbour is actively playing SuperСity but hasn't sped up or collected buildings in your city during the last 5 days (or has, but you've already helped him back).
Grayed out icon means that this neighbour hasn't launched SuperСity in the last 5 days. You can't visit cities of such neighbour, you can only call him back to the game. You both will get a gift if your "sleeping" neighbour comes back within 5 days. You can also call all your "sleeping" neighbours back at once tapping the "Invite all" button.