Since we are all talking about this, I'll post
this link. I didn't realize just how few people would know this law among those who you'd expect: fire fighters, ex-law enforcement, acreage owners, etc. An interesting part of the law is that an obvious crop present or just cattle make entry without permission unlawful, even if from the air. If a hot air balloonist comes down in the middle of your grain field, they are trespassing it would seem.
My reason for purple painting my trees is to prevent somebody from being able to say they did not know the property was posted. I'll use a combination of purple paint and signs. Every year I have to run people off my property; some are neighbors and others are people I don't know who think they can get out of their boat on my shoreline and then go "exploring" at will. I went to Walmart yesterday and bought a quart of bright purple exterior latex paint and a couple of cheap brushes. This weekend, I plan to take my grandson and go out marking trees. We are gonna mark the dead trees with bright international orange and several property line trees with purple. It'll be an adventure.