I think you may be referring to the little round dot on the bottom left of the area beside where your username is shown against any posts you write?
If so, the colours indicate as follows:
Grey (Blue) - user is currently not logged in to the forum (but could still be viewing as a guest)
Green - user is currently logged in and using the forum to read / write posts
Red - user has chosen to go into ‘invisible mode’ which means they are logged in but do not show to other members as being logged in.
A user who is in ‘invisible mode’ will see a red dot against their posts, and won’t appear in the
Who’s On-line list for others to see. For them they will only see a grey dot against any posts that user has made while in this mode.
You currently appear to be in invisible mode from looking at your user profile. You can change this setting at any time by clicking on User CP and selecting ‘Edit Options’ which is the 8th header down, once changed on the next page, don’t forget to scroll to the bottom and hit ‘save’
One final thing, ‘invisible mode’ is only available to subscribed red members, it’s an extra bonus option given to those who choose to be subscribers.
Hope that helps?