Colours of the Day?

[SuperSan]

What does it mean to "give some the colours of the day"?

Thank you much!!

[Quizmonster]

During World War II, I believe, allied aircraft used a "colours of the day" system to indicate to ground gun-batteries that they were friendly. In other words, it was a sort of code or password. Presumably, they were changed every day so that the enemy would not cotton on.
I imagine, therefore, that "to give someone the colours of the day" nowadays means either that you tell them what is required in order to get by or that - the other way around - you are covering your butt from criticism!

[dr john]

Undercover cops working in gang districts in America have the same system to avoid accidentally shooting each other.