If the ScreenRuler colour overlay does not span over all of your monitors, read below for instructions on how to fix this. This is also related to ClaroRead Mac Toolbar Does Not Reappear After Minimising.
Click on the Apple logo in the top left of your screen
Select System Preferences
Select Mission Control
Uncheck Displays have separate Spaces
Log off or restart your computer
Launch ClaroRead – the toolbar will now reappear after you minimise it.