Visualising changed/added/removed Git lines in IntelliJ IDEA

You need to set up version control for your project. You can do that in "File" -> "Settings..." -> "Version control" -> then set "VCS" to "Git" (or any other supported VCS). Once you have done that, the editor shows color bands for changed lines (green for added, blue for changed, red for removed). See also the IntelliJ manual page.

If the gutter marks are missing this could be caused by plugins. I was able to get them back by enabling in settings (in Webstorm):

Editor > General > Highlight modified lines in gutter