Carotene

Nutrient responsible for the red colour in Koi. Nishikigoi absorb carotene as a nutrient in order to live. When the carotene stays in the fat of the body surface it appears as a red stain. Because Shiro Utsuri don't have any red in their patterns carotene fed to this variety can cause red staining to appear.

(Courtesy of the excellent Book Kokugyo Vol II)