Green Bars in Visual Studio 2010

They show lines that you have changed.

See here for more details and a table showing what the different colours mean:

+-----------+------------------------------+-------------------------+
| Marker    |     Different from           |    Different from file  |
|  colour   |       file saved on disk?    |      that was opened?   |
+-----------+------------------------------+-------------------------+
| Nothing   |        No                    |       No                |
| Yellow    |        Yes                   |       Yes               |
| Green     |        No                    |       Yes               |
| Orange    |        Yes                   |       No                |
+-----------+------------------------------+-------------------------+

Edit: Blue indicates that the line was changed, saved and then reverted by undoing.
To see the entire list of colour schemes, go to Tools>Opitons>Environment>Fonts and Colors.(or Ctrl+Q.then type "Fonts and Colors")

Yellow - source code that has been changed since you opened the file, and that hasn't been saved.

Green - source code that has been changed since you opened the file, and that has been saved.

IN SUMMARY...

Start out blank.

Make a change = Yellow.

Save the change = Green.

Undo = Orange.

Change the line again = Yellow.