Something Blue

In real world Earth, "something blue" was made up by a 19th century poet because it rhymed with "new," it was never a major tradition before that poem.
In Scarterra the color blue is commonly associated with Mera and Mera is the goddess of families, it is commonly considered good luck to invite Mera's blessing on a wedding.  Blue is often but not always incorporated into what the wedding decorations.  Less commonly the bride or perhaps the groom carries a blue token.