The the Lakota, the four sacred colors represent the four races of humanity.

"For the Lakotah, black, red, yellow and white are four of the sacred colors. They represent the four races of people, and when you mix those four colors together you get brown, the color of the other race, and the color of our Grandmother the Earth."

Russell Means
Oglala Lakota

Photo: Yellow Mounds section of Badlands National Park, SD, May 16, 2016