Ah, sorry, I mean the Folders section of the COP Catalogue; not the image folders within the finder. The colour tags inside COP are not the same as the ones in Mac OS and so aren't visible there.
And indeed even at that I got it wrong - to see all reds in your Catalogue, go to
Catalog Collections
All Images
which will show you everything
Then filter to reds only
Select all (Cmd-A) - and be very sure you have now selected just those you want to delete!
Right-click the selection and select "Move to trash"
You'll get options to either just delete the files from the catalogue and leave the masters in place, or delete the masters as well