Add food coloring. The skin of sweet potato is mainly made of round glutinous rice and glutinous rice flour, and starch and boiled water are used as binders, so the color is usually white, but some people add food pigments to it to make it have other colors, such as yellow, purple and green. So potatoes are brown because of the addition of food coloring. Sesame potato, also known as sesame paste, is a traditional food, which originated in China.