A colored bandana or other clothing item worn by undercover cops. Every day this color changes so cops in uniform can tell an undercover cop without having them pull their badge. This allows undercover cops to maintain undercover status while providing protection in any instance of uniformed involvement.

Got the covid-19 vaccine

reffering to the color of a girls underware

A game some guys (doesn't matter what age) play where they try to see the color of a pretty girls thong(underwear) for the first time that day.

UPDATE:

Color of the day Was started in Ellet By Zack Rodgers And Jessi Nagle, Color of the day reffers to the color of a girls underware (usually the color of a thong)