Purple icon means that this friend has recently given help to your city (no earlier than 5 days ago). "Purple friends" are displayed first in the list, so that you could see who's helping you. If you help a purple friend in return, their icon will turn blue.

Blue icon means that this friend is actively playing Supercity, but hasn't given your city help in the last 5 days (or has, but you've already helped them back).

Grayed out icon means that this friend hasn't launched Supercity in the last 5 days. Unlike purple or blue icons, which are displayed in the order of their levels, from highest to lowest, "sleeping friends" are sorted by the date of their most recent visit to the game.

If you click on a grayed out friend icon, you'll be able to "call" them back to the game. This sends them a Facebook notification, and you both will get a gift if your friend comes back.