Changing your colours means (in Navy terms) to change your flag.
In the Middle Ages it was used for Heralds who changed their tabards inside out when their enemy began winning.
In the army they wear DPM – Disruptive Pattern Material. Like Tigers or Zebras or whatever.
TURNCOAT comes from the Middle Age Heralds turning their coats.
Double Down applies too. Once you turn you cannot RE-TURN. Unless you want the hounds of hell on your bum.
I have such a vivid memory of my late father seeing Boy George for the first time on Top of the Pops.