Much as I respect Lomax as a collector I've never heard of red being used to denote fidelity!

Many folk songs however refer to blue ribbons worn by either gender as a symbol of fidelity/betrothal
An ancient custom - dating from Biblical times, I believe, reflected in the modern wedding custom of 'something blue' - and used in such songs as "The Trees they do Grow High" and the nursery rhyme "Johnny's So Long at the Fair"
- He promised he'd bring me a bunch of blue ribbons.
To tie up my bonny brown hair

Then there's always the traditional broken token - but that's another thread :)