The color combination, yellow and black, is considered as the most easily identified attention getter by psychologists in the graphic arts industry. 'It had to be simple enough to be easily identified by children, non-English speaking persons or others who may not be able to read. The sign had to meet the psychological requirements of simplicity, easy identification, retention and arresting color combination.' 'In awarding the contract for design of the sign to graphic arts studios it was designated that the services of a psychologist be obtained to recommend a visual symbol that could be easily identified and remembered. The following description is taken from a draft version of a fact sheet entitled 'National Fallout Shelter Sign.' In 1962, contracts were negotiated for the production of 400,000 aluminum outdoor signs and one million steel signs for indoors. Unlike this example, these signs often had yellow arrows below the words 'fallout shelter' to indicate the direction to the shelter. It was intended to only be used with federally approved shelters. The above version of the national fallout shelter sign was introduced to the public by the Defense Department on December 1, 1961. The Office of Civil Defense originally intended fallout shelters to use the radiation warning symbol (yellow background with a magenta circle in the center of three magenta blades) but this idea was rejected because a fallout shelter represents safety whereas the radiation warning symbol represents a hazard. Use inside facilities to mark access routes to shelter areas and the shelter area itself.' The same document describes the sign shown below as follows: 'Interior sign, Type II, size 10' x 14', galvanized steel, 0.018' thick, with partially reflectorized surface and overlays. Used to mark facility entrances.' The interior signs were made of steel.

The 'Posting Fallout Shelter Signs Handbook' FG-C-8.1 (June 1964) describes the sign below right as follows: 'Exterior sign, Type I, size 14' x 20', aluminum, 0.025' thick with partially reflectorized surface and overlays.

Civil defense fallout shelter plans