The blue icon means you're an admin of the board (or organization). The green/yellow/grey icon means you're online/idle/offline.

It indicates your online status.

Green: Online and active, Yellow: Online, but inactive, Grey: Not online

When someone is online it means that they're connected to Trello, but they're not necessarily looking at the same board as you.

Thanks for explaining that. I would have expected mouseover text for those like there is for the blue square.