I've set up a master page that contains frames that have "master page overrides".
I can override the text with no issues.
When it comes to overriding a graphic (in this case I have a grey rectangle that I want to change the colour of), it seems to add a copy above all the other layers.
What am I doing wrong here? As i'm guessing the override should allow me to adjust the original graphic just like it does with the text. (When overriding the text I've been using ctrl+shift+click)
You overridde any master object the same way. Perhaps the stacking order isn't waht you expect because all objects on the document page are above all master objects on the same layer. That could move waht you thought was a background object forward.
Using layers on your master page may help this work more predictably. I always put page numbers on the top layer for this reason, with most document content on the lower layer.