I find that you are doing purple fence posts to be so delightfully appropriate.  Your color... and one the locals most likely know to respect.  I guess the tradition goes back.

My moms side of the family were country folk.  I used to visit my grandfather in rural NC and we would take a couple cane poles for fishing.  We fished in a creek on his property, but that was on the edge of his neighbors property.  My grandfather, a man of few words pointed out a post with a purple top and asked me if I knew what it meant.  Nope, I said.  And he told me, it's a friendly way to warn people "if you go on that land they'll shoot ya" (the way my grandfather said it was more like "shoochee" lol).