49 C.F.R. § 172.327
A Bulk packaging used to transport petroleum crude oil containing hydrogen sulfide (i.e., sour crude oil) in sufficient concentration that vapors evolved from the crude oil may present an inhalation hazard must include a marking, label, tag, or sign to warn of the toxic hazard as follows:
(a) The marking must be durable, legible and of a size relative to the package as to be readily visible and similar to the illustration shown in this paragraph with the minimum dimension of each side of the marking at least 100 mm (3.9 inches) as measured from the outside of the lines forming the border. The width of the border forming the square-on-point marking must be at least 5 mm. The marking must be displayed at each location (e.g., manhole, loading head) where exposure to hydrogen sulfide vapors may occur.
(b) The border of the square-on-point must be black or red on a white or other suitable contrasting background. The symbol must be black and located in the center of the square-on-point and be clearly visible as follows:

[76 FR 3367, Jan. 19, 2011, as amended at 80 FR 1150, Jan. 8, 2015]