Ah I think I can explain this one.
Its a simple device. When you log in all the threads you havent read are RED..or in actual fact there is a red logo ...this indicates that some messages have been posted on that thread since you last looked at it.
When you read the threads they go GREY...
which indicates you are up to date.
Other threads will also be RED and when you read them they will also go GREY.
In other words at a stroke it shows you overall the ones you have read and the ones you havent read, so you dont have to click unnecessarily on the same threads.
Good isnt it. Its not new though its always been there.
Its only for members. If you dont log in at all...all the threads are forever GREY logoed.
So there you are...hopefully that's clear...
