Why are school buses yellow in the U.S.?

There used to be many different bus colors in the United States. But in 1939, after a nationwide survey, Yellow, or “National School Bus Glossy Yellow,” as it is officially called by the National Highway Traffic Safety Administration, was chosen for its high visibility and the way it emphasized the bold, black writing that would be on the side of each bus to denote its respective school district. This yellow color is an important factor for vehicles that travel during early morning and late afternoon hours.

Source: Reader's Digest