The white flag is actually an internationally recognized protective sign of truce or ceasefire and also a request for a certain negotiation. Other than that, it is also used to symbolize surrender since it is often the weaker military party that requests negotiation. A white flag signifies to all that an approaching negotiator is unarmed, with an intent to surrender or a desire to communicate.
Tim Marshall who is the author of “Worth Dying For: The Power and Politics of Flags” talk about why the white flag is used as a symbol of surrender worldwide. He said, “It’s instantly recognizable. You don’t need to read and write. You don’t need to know much about history. You see that flag, you know what it is. Immediately, right across the world”.
Historians agree that the color selection was just a matter of convenience. Since white cloths were quite common and always handy in the ancient world, this is probably the case of troops having to improvise with the materials they had on hand. In addition, white is highly visible in the heat of battle and against most natural backgrounds so it made sense to use them at the battlefield.
In addition, the color white has long been associated with death and mourning in China, so its soldiers may have adopted white surrender flags to show their sorrow in defeat. Today, a white flag is considered a symbol not only for surrendering but also for the desire to call for a parley.
Sources: Insider, YouTube Business Insider.