About Hangouts availability
You can start a Hangout with anyone you want, anytime you want. When you start a Hangout with someone on your desktop, you can look at the person's profile picture in your Hangouts list to find out whether the person will be notified about the Hangout immediately or later on.
- Profile picture with a green bar underneath it: These people will be notified about new messages or video call invitations immediately. They will be shown with a green dot next to their name, indicating that they are available, in the Chat list of people who are using Google Chat, Google Talk, or a 3rd party chat app.
- Profile picture without a green bar underneath it: These people are likely on a mobile device, have Snooze Notifications enabled, or are idle on their computer. They will be notified about messages and video call invites, but they may not see it immediately. These people will be shown with a yellow dot next to their name, indicating that they are "idle", in the Chat list of people who are using Google Chat, Google Talk, or a 3rd party chat app.