Is dark brown stool mix with a little yellow brown normal? What causes it?

Brown /yellow brown. I would not worry. Stool is a combination of bacteria, food residuals,bile . Diet affects color-carrots/ sweet potatoes more yellow. Butterfish orange/ oily .Colors to worry are red current jelly/ black tar ( blood) and chalky whitish ( gallbaldder) and oily gassy stool(pancreas) and if constantly light yellow. f/u/ w/ doc http://www.mayoclinic.org/stool-color/expert-answers/faq-20058080.
Can be. Soft Tan colored stool often is due to more rapid transit of intestinal contents and it is possible that some feces travelled through the intestinal tract at different speeds leading to the two different colors in the stools. Certain foods,like fruits and salads, may have faster transit through your intestinal tract and may cause actual diarrhea in some people while lower residue foods don't.