Purple isn't just for Prince fans.  It's also used to keep hunters and hikers off of private property. 

If you see purple paint marking a fence or trees while you're out in the woods, you need to turn around in Indiana.  Purple paint is a sign for no trespassing in many states.  The "Purple Paint" bill passed the senate on May 24th of 2006 according to MichiganVotes.org,

To establish that a property owner can “post” his property to indicate that trespassing is prohibited by placing purple paint marks on trees or posts not more than 100 feet apart and readily visible to a person approaching the property. Each paint mark must be a vertical line at least 8 inches long, between 3 feet and 5 feet above the ground.

The reason behind this is simple.  Property owners are constantly having to replace vandalized or stolen no trespassing signs.  That can be a very expensive problem if you own a lot of property.  The proposed solution would allow property owners to spray paint fence polls, fences or parts of trees in order to indicate their property line.

However, the bill stalled in the house of representatives according to Mucc.org,

In other states, a purple paint mark on a tree has the same effect as a "No Trespassing" sign. The idea was proposed in Michigan by legislation in 2006, but stalled in the House of Representatives after being passed in the Senate.

It's not clear why a bill that passed the Senate with a 35-2 vote failed at the House.

But, if you see purple paint markers while out in the woods, it's better to be safe than sorry.  Those purple markings are more than likely property/no trespassing markers.