|Hi Street Cynic....|
The blue and pink colors are the messages that you see when you look at the whole page of messages - that differentiates between read and unread messages.
About the hexagon icon, I see that Pine Valley answered that one.
Last question about all replies in one place: To my knowledge, there is not a simple list of them, but if you look under the "MAIL" icon at the top of the page (next to the SI icon), you will see all of your replies listed there individually - in the drop-down menu. You can then click on the link for each reply and it will take you to it - wherever it is.
I hope I have been able to help. IMO, SI is the most convenient and user-friendly site there is on the internet.