- Green: Available
- Yellow: Idle. Your status automatically changes to idle when you're away from your computer for 15 minutes. You can't manually select to appear idle.
- Red: Busy. This lets your contacts know you're busy and don't want to be interrupted.
- Grey: Signed out of chat, or Invisible. (If you set your status to 'Invisible,' you'll appear to be signed out even while you're actually signed in.)
- From the Set status here drop-down menu, select Custom Message... (you can set custom statuses when you are available and when you're busy).
- Enter your custom message in the box, and press Enter.
If you also use the downloadable Google Talk client or the Google Talk Gadget, your status will be shared by the applications, as long as you're signed in to all of them. Please note however, that if you've set your status to 'Invisible,' and sign in to the Google Talk client, you will not be kept 'Invisible,' as the Google Talk client doesn't currently have the 'Invisible' status feature. Keep in mind that you can sign out of chat in Gmail, and still be online through Google Talk and vice versa.
More information: