Bright colors like yellow work well. I would avoid colors that may be hard to see depending on the weather, like brown or dark blue, or colors that may be hard to see on certain species of trees, like white, gray, or green. As for longevity, the type of paint is more important than the color. Paint companies make actual Boundary Line Paint that is designed to last more than 5 or even 7 years. I would check out Ben Meadows or Forestry Suppliers as a start. Also check out the Paint My Boundaries activity within the Privacy and Trespassing Privacy section. You'll find more information there.

NCP coatings is another brand that I have used. Their boundary line paint has lasted over 7 years. Usually by year seven it is time to re-paint to keep the paint showing up well. In the fall the yellow and red boundary line paint can sometimes blend in with the fall leaf colors.