49 CFR 172.304 - Marking requirements.
(a) The marking required in this subpart -
(1) Must be durable, in English and printed on or affixed to the surface of a package or on a label, tag, or sign.
(2) Must be displayed on a background of sharply contrasting color;
(3) Must be unobscured by labels or attachments; and
(4) Must be located away from any other marking (such as advertising) that could substantially reduce its effectiveness.