Dark brown is best as it tells you the bile which has not been readsorbed into the bile pool is present in an adequate amount. Years ago I would pass dark brown stools after eating red meat but would pass lighter brown stools when I ate fish or poultry.
Have you noticed any signs of jaundice? This would mean that the breakdown products of bile are building up in your blood . I think a Dr would test you for blood bilirubin, urine urobilinogen and elevated liver enzymes. This is valuable information as are the other tests you would get to see if you have a virus or toxin affecting the liver and Gallstones or sludge in the bile system. If you choose self treatment then you have a better idea of what you can do.