It comes from legends about Red Croatia and White Croatia. Constantine VII Porphyrogenitus, an Eastern Roman Emperor in the 900s talks about inviting the tribe of "White Croatians" to be part of the empire, when they migrated from Bavaria & Krakow, because they were prone to allying with Franks or Turks and that made them an unpredictable military problem that had to be subverted or controlled if they were gonna live on the close side of the Carpathians.

Later around 1300, Red Croatia was invented by a clergyman working under the Archbishopric of Split either to expand Church claims or as a political favor for the Šubić family to justify claims of conquest, expanding the definition of what was "core" Croatian territory to include lands south of the "White Croatians" discussed by Constantine VII Porphyrogenitus.

As centuries passed, all this very shakey and unconfirmable history became national legends everyone took for granted

hi! you might consider x-posting this question to the larger pool of flag afficionados in /r/vexillology