Speed-limit signs in the United States typically have a white background.
A U.S. speed-limit sign is a regulatory sign: it states a legal maximum speed for the road, lane, or area under the applicable jurisdiction. The standard design is a white rectangle with black lettering, usually reading “SPEED LIMIT” above the numerical limit. White is therefore the background color, while black provides the main text and numerals.
This differs from the Vienna Convention style used in much of the world, where speed limits are commonly shown as black numerals inside a white or yellow circular sign with a red border. The U.S. rectangular design is associated with the Manual on Uniform Traffic Control Devices, which sets national standards for traffic-control devices.
The word “typically” matters. U.S. jurisdictions may use variations, including signs labeled “MAXIMUM SPEED” in places such as California and Alaska, or special signs for minimum speeds, school zones, work zones, trucks, and variable conditions. A yellow sign near a speed limit may warn about a curve or other hazard; it does not change the standard background of the regulatory speed-limit sign itself.