I've recognized this a few times in Texas. Trees were more or less "professionally" painted purple. It appeared totally random to me but was too frequent to just take it as a plain tag from the owner (like, "I especially like this tree, I'll make it purple"). Does anybody know what the paint signals?
Purple paint is a common means of indicating private property boundaries in a number of US states. Some states use alternate colors such as orange or blue. The paint may be considered the equivalent of a "no trespassing" sign, but is considered more practical for marking property lines that may extend for miles.
There are usually requirements associated with the markings. For example, Texas law (Penal Code §30.05) requires:
(i) vertical lines of not less than eight inches in length and not less than one inch in width;
(ii) placed so that the bottom of the mark is not less than three feet from the ground or more than five feet from the ground; and
(iii) placed at locations that are readily visible to any person approaching the property and no more than:
(a) 100 feet apart on forest land; or
(b) 1,000 feet apart on land other than forest land
Ref: Purple Paint - Texas No Trespassing Marking from geocachingaustin.com