yes, the dyes in lower end dog food like kibbles and bits will affect stool color.
please don’t feed a medium breed dog puppy food for small breeds. the nutrient amounts are incorrect for your puppy and can cause health problems. try to feed something that has no corn and has meat for the first two or three ingredients.
i would also wonder why you have a puppy 2 weeks early. puppies should not leave their mother and littermates prior to 8 weeks of age. is there any way you can take your puppy back to the breeder for 2 more weeks?
Lethargy is not normal! He needs a vet.
I’d keep an eye on him, if he’s lethargic for more than a few hours or other symptoms appear, definitely get him to a vet.
Thank you.