What's with the status icons on the far left column?

[beer]

This has bothered me for some time...what is up with the difference between the blue and yellow icons? I know red icons looking like a lock means the thread has been locked, but what's the difference between blue and yellow?

 

[konichiwa]

Blue = new since last login
Yellow = old since last login

 

[loogie]

it indicates if there has been a new post in that thread.