I probably wouldn't do borders. However, I'd wait until the top was done before making that final decision.
The white parts depend on what kinds of colors I'm using. If my scraps were brights, I'd go with white or WOW prints. If my scraps were darker, more in the style of civil war repros, I'd use naturals or tans to soften it.
Are you using scraps or yardage? I take it the diamonds are made up of your "stamps"? That will be so neat!