The colours do not really mean that much.
If the date "Last login at" is green, the user is probably still logged in as he has not log out yet. But he may also have closed his browser without logging out.
Please note that sessions are usually closed after max. 5 hours, even if a user did not log out.
If the "Logout date" is shown in Red, the user has logged out at that date and time.
The "Last action" date shows the last ConfTool page access of the user.