Signs used to warn of potentially infectious materials should have a _______.

The use of a fluorescent orange or orange-red background with a black "biohazard" symbol is the recognized standard for warning signs that indicate the presence of potentially infectious materials. This specific color combination is designed to ensure maximum visibility and immediate recognition of the hazard by anyone who may come into contact with the area or materials in question. The vibrant colors attract attention and serve as a clear signal, indicating that caution is required due to the potential risk of exposure to bloodborne pathogens.

This standard is established by regulatory guidelines, such as those from the Occupational Safety and Health Administration (OSHA), to maintain safety in environments where biological hazards may be present. The black "biohazard" symbol is universally understood, further reinforcing the warning conveyed by the sign.

Other color options, while they may have their own uses in different contexts, do not convey the same level of urgency or clarity regarding biological hazards, which is why they are not appropriate for this specific signaling purpose.