The official name for the color was National School Bus Chrome. Later re-formulations minimized or eliminated toxins such as chromium and lead but retained substantially the same color.
“In April, 1939, [Frank W.] Cyr organized a conference… that drew transportation officials from each of the then 48 states, as well as specialists from school-bus manufacturing and paint companies. At that conference, funded by a $5,000 grant from the Rockefeller Foundation, the participants established national school-bus construction standards, including the standard color of yellow for the school bus…
“The color was selected because black letter[s] on that hue was easiest to see in the semi-darkness of early morning and late afternoon.”
Excerpt from Cyr’s obituary at http://www.columbia.edu/cu/record/archives/vol21/vol21_iss1/record2101.36.html . Learn more about Cyr and the school bus standards at the link above and at https://www.quora.com/Why-are-school-buses-yellow-1 .