Section 30.05 of the Texas Penal Code defines criminal trespassing. This law also describes how to give proper notice of forbidden entry. This includes the use of purple paint marks according to very specific rules. According to Sec. 30.05, the purple marks must be:

• vertical lines at least 8 inches long and at least 1 inch wide
• placed so that the bottom of the mark is at least 3 feet but no more than 5 feet from the ground
• placed at locations that are clearly visible to any person approaching the property and no more than:
  • 100 feet apart on forest land, or
  • 1,000 feet apart on land other than forest land

You can find more information about trespassing on the Texas Parks and Wildlife Department's FAQ on properly posting your property.