Thank you for your query.
It is very common for the stool of babies to change colour after you introduce different food items. I would not worry about it.
over chin, neck and tummy, it shall have to be observed as how they progress. In case you introduced top milk for the first time in the form of oats cereal and fruit mix cereal then one needs to exclude cow milk allergy
. However my advice would be to wait and watch as there can be other reasons for the rash. If need be consult a doctor.
I hope I have answered your query to your satisfaction.