Hello, @pete-wright,
I suppose that you, probably, use the ChangeMarker plugin, which is responsible of the colouring. Refer to the page :
http://www.off-soft.net/en/apps/notepadpp/notepadpp-changemarker121.html
This plugin may be downloaded, from the address :
http://www.off-soft.net/en/notepadpp-custom#tag-change-markers
The latest version is 2015-01-13, v1.2.2_Rebuild
In addition, when new changes are saved, the dark orange colouring is changed into a pale green colouring
Note that you may modify the colours of not saved and/or saved lines, easily :
Choose the menu option Settings > Configurator…
Choose the Change Marks language, at the end of the drop-down Language list
You may modify, either, the colours of the Changes: Saved and Changes: Not Saved styles
Colouring may have four different forms :
A vertical mark, in the Line Number margin, for the changed or saved line
A vertical mark, in the Bookmark margin margin, for the changed or saved line
A vertical mark, in the Change Mark margin, for the changed or saved line
A complete highlight of the Unsaved or Saved line
Best Regards,
guy038