Quote taken from FAQ
~MULTIPLAYER GAME COMPLETION PERCENTAGE
I see a colored ball with a checkmark in it on my account and other players in a multiplayer lobby. What is that?
The colored ball you see next to player names in multiplayer lobbies is their Multiplayer Game Completion Percentage. It represents the percentage of the time the player finishes a multiplayer game. Green represents excellent completion percentage, Yellow represent good completion and Orange represents below average completion. The value are determined by the player's last 100 multiplayer games.~