What do the different user icons mean?

Base icons:
Green Green
Standard user icon (no particular attribute).
Grey Grey
The user is in away mode.
Yellow Yellow
The user represents a bot the hub uses to issue automatic commands.
Icon modifiers:
Red bricks Red bricks
The user cannot be connected to, for he is in passive mode and does not support NAT traversal.
Red circle Red circle
The user has no free slot available.
Yellow key Yellow key
The user is a hub operator.