This answer to the question What is the purpose of putting a red flag on the dome of Hussain ibn Ali(as)'s shrine? states that if Arabs were at war with each other, during the Haraam months they would ceasefire (temporarily). To notify the opponent about it they would raise a red flag on the dome of their commander's marquee. And would resume thereafter.
Is that correct?
If so, how, when and why was this custom adopted?