I took Iron supplements after donating blood for many years until I grew too old. I never noticed any such color change, but then again I never did a detailed examination of what comes out.
People really should be aware of the details of living. Poop is normally brown because all our food comes from dirt and dirt is brown. But if you eat a lot of something with strong color, your poop might be that color. A big serving of fat causes yellow or white poop depending on what kind of fat. Bleeding in your stomach causes poop that looks like coffee grounds, but bleeding further down the pipe gives spots of bright red. If you eat sweet peppers, those also cause spots of bright red, so a doctor has to ask about that if he sees red during an examination.
You hear a lot about fiber. Fiber does exactly two things: it absorbs water and it takes up space. But if you don't get enough, what happens is very unpleasant. Spider veins in your legs is one possibility, bellyache is another. If you get enough fiber your poop will be soft but not loose. Two slices of whole grain bread every day is enough. That or veggies with every meal.
These books are old, but still the most popular introduction to the subject: https://www.amazon.com/Adelle-Davis/e/B001J3RULO
I paste this only because I am, in general, a contrarian:
Adelle Davis used to say that she never saw anyone get cancer who drank a quart of milk daily, as she did. She stopped saying that when she died of cancer in 1974, leaving behind her a trail of ten million books and a following that was large, devoted, and misinformed.
That's nice. Nutrition is a young science, barely a century old. It is to be expected that a lot of stuff we think we know is not so. For example, the official opinion about butter has reversed several times in the last century. Most people know nothing of the subject except what they heard from their grandparents.
And nobody else has found a cure for cancer either.
Not Sure 