"An edict during the persecution of Buddhism under Emperor Wuzong (r. 840-46) proscribed black robes (the preferred Tang color for Buddhist robes), linking them to 'pigs, dogs, black donkeys and oxen, and the like...because the Taoist priest wears yellow.'"
Bogel, Cynthea J. With a Single Glance: Buddhist Icon and Early Mikkyō Vision. Seattle, WA: University of Washington Press, 2009. pp. 135.