When comparing Revisions, Blue is NEW, Red is DEAD, meaning that objects that are colored in Blue exist on the newer revision (or whichever the Plan you choose to Overlay on top of the current plan), but do not exist on the older or base plan. Objects colored in Red have been removed, meaning the objects exist on the current/earlier/base plan but not the newer Revision (again, or whichever Plan you chose to use as the Overlay).
The Overlay function is designed to show you differences in the Plans at reasonable zoom. Overlay will show you pixel-level changes if you zoom in extremely closely. Use Overlay to identify changes on the Plan, not to compare high-zoom details.