As "toku" (virtue) governs the other five virtues, it is presumed that it was allocated 'purple' since that had been valued as the color of the sovereign (emperor) since the Han dynasty (Tenmonshi of "Kanjo" (Historical records of the Han Dynasty)).
徳は、五常の徳目を統べる意があることから、漢代以降、帝王の色として尊ばれた「紫」を充てた推測できる（『漢書』天文志）。