Warning Signs in the MUTCD are BLACK an YELLOW. See the MUTCD for more information. https://mutcd.fhwa.dot.gov/pdfs/2009r1r2/part2c.pdf
Department of Transportation agencies use 3M DG3 FLUORESCENT YELLOW for their warning signs, as well as many city dpw's and county road associations. 3M High Intensity Prismatic is widely used as well.