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”)