A green icon indicates that the person is available for phone calls.
A yellow icon indicates someone is currently calling to the person or that the person is making an outbound call but the other party has not yet answered. In this case, you have the option to help out and pick up the current incoming call. If you wish to do this, mark this extension in the list, right-click, and select Pick up.
A red icon means that the person is currently on a call.
A grey icon appears when the person has turned on a notification profile and is not available for a call. If you want, you as the operator can call anyway. See "How do I dial an outgoing call? "for further info.