SecretLeeds - History, culture and architecture in Leeds

Sorry if I am missing something?
What is the significance of the different coloured 'balloons' - some with projections in the left hand column.

I think that a faded green balloon behind a gray one indicates you have read the most recent post in the thread/forum. A stronger green balloon fronted by a brown one means you haven't seen the most recent addition(s). All assuming you are (a) logged in and (b) have cookies enabled for the site