Yellow stool is not 100% sure sign of low bile flow.
You may have enough bile, and still have yellow stool. Yellow color stool is more of the sign of dysbiosis, or if you eat white food, it may be a sign of the color of the food your eat.
Natural bile color is green, blue-green.
It changes color during digestion, thanks to the work of intestinal microbiota (many different species of life, bacteria, fungi, archaea) https://en.wikipedia.org/wiki/Gut_flora