Thanks to cknoor for pointing me towards a tutorial, will link it later!

This is what I ended up doing, for OBS 64bit 0.16.2

• Open a Paint Window
• Add a Window Capture and choose the Paint window.  Make sure this is the top layer on OBS
• Click on Filters, and set the Color Key to those items shown
• Also in Filters, set Reduce until you can’t see the top window and the scroll bars – this will depend on your monitor (I think, I’m no expert!)
• Choose the size of the Paint window to match the OBS setup – this is just trial and error.