Original: An undercover police officer will wear a headband, wrist band or other conspicuous clothing in the "color of the day"; which is relayed to officers at the station house or could formerly be heard on very early morning radio in New York on station WBLS.
Simplified: An undercover police officer will wear a headband, wristband or other piece of clothing in the same color as the "color of the day"; this color is told to officers at the station house before they start work.