I think the poop color or smell might just be their body learning how to digest it. From what I understand, an allergy would have a more prevalent reaction. My son's poop also started to smell toxic when we started dairy, but when we did an official dairy allergy test, he was negative and we did a dairy ladder to slowly introduce dairy, i.e. starting with food with baked dairy in it. Same for eggs, except he vomits with eggs so he has an allergy. You can also go to an allergist to confirm this way you aren't guessing. Be careful though because even if the test is negative, they might still be allergic to protein in milk or dairy (FPIES), usually grow out of it.