49 CFR §172.527

1. (a)Except for size and color, the square background required by § 172.510(a) for certain placards on rail cars, and § 172.507 for placards on motor vehicles containing a package of highway route controlled quantity radioactive materials, must be as follows:
2. (b)In addition to meeting the requirements of § 172.519 for minimum durability and strength, the square background must consist of a white square measuring 14 1/4 inches (362.0 mm.) on each side surrounded by a black border extending to 15 1/4 inches (387.0 mm.) on each side.