Several states have a "Purple Paint Law" which allows property owners to mark their property and post it as "No trespassing" using purple paint. Why purple paint? For one, people often steal, destroy, or remove No Trespassing signs. For another, purple is a color that is visible, but not often used to mark anything outdoors, like utilities or surveys for instance.

I have seen several websites, such as amw.com, which lists Montana as a purple paint state.  I have never personally encountered this in Montana, but since our state is so massive and I don't wander around much, I thought I should do some more research.

The relevant law I found says nothing of purple paint; however, it does allow property owners to post their property with orange paint.

To provide for effective posting of private land through which the public has no right-of-way, the notice provided for in subsection (1) must satisfy the following requirements:
(a) notice must be placed on a post, structure, or natural object by marking it with written notice or with not less than 50 square inches of fluorescent orange paint, except that when metal fenceposts are used, the entire post must be painted; and
(b) the notice described in subsection (2)(a) must be placed at each outer gate and normal point of access to the property, including both sides of a water body crossing the property wherever the water body intersects an outer boundary line.

If you've encountered any of these purple paint markings in Montana, leave me a comment and let me know.  Otherwise, remember that if you see a fluorescent orange marking while your out hiking or hunting, you might want to turn around and go back the way you came.