I grew up with purple paint being a common item to let others know of your property lines and trespassing. Texas put this into law back in 1997, so is nothing new, but apparently new to people that don't own land. I've attached a photo of both a sign about the law and the purple paint. Usually you will not see the sign, just the paint. There are rules about how high this paint is to be on the tree or fence, how wide it is and how tall.
Of course, not all land owners apply the paint the same. I run across the purple paint all over Texas, both on pine trees in the east and on fence posts in Central and west.
Also, for those that travel, where there are at least five states that also use purple that are more than 10 that use orange or lime green.
Many states require the property owner to post a notice to stay off the property. Posted notices must be in purple paint in Arkansas, Missouri, Texas, Maine, and Kansas. Posted notices in other specific colors are required in Arizona, Idaho, Louisiana, Maine, Maryland, Nevada, Tennessee, Utah, Virginia and Wisconsin.
Read more: Criminal Trespassing Laws | eHow.com