Does your stool get darker if you don't have a bowel movement for 2_3 days. not black but reg and dark brown together?

Change in stool color depends mostly on your diet, less likely the stool transit time. Of course, some diseases can change to stool color, such as bleeding from your upper GI tract can make the stool dark black or the stool becomes light or even white when the bile duct is obstructed.

Stools that pass faster through the intestinal tract tend to be lighter in color while slower passage does result in darker brown and firmer stools.