The new law went into effect on July 1. The goal is to make it easier for Indiana landowners to mark their property and prohibit trespassing.
“The Purple Paint marking has to be at least three feet to five feet off the ground, no more than five to the bottom of the marking. The marking has to be at least eight inches long on a tree and one-inch wide.”
http://www.wdrb.com/story/38560286/indiana-purple-paint-law-in-place
here we can mark posts as well as trees as often there are no trees west of Fort Worth , not sure about Indiana