When playing online, there is a circle next to your gamertag that changes colors. Here are the colors and what they mean:
Green: A green circle signifies that you are a clean racer; meaning you race the majority of the time without running into other players and scenery.
Yellow: A yellow circle is about average; you race as clean as you can but still come into contact with other drivers/scenery from time to time.
Red: A red circle signifies a dirty racer; someone who constantly wrecks into other drivers/scenery. These are drivers you may want to avoid.