Student or Learner
Baby's poop color change from brown (unnormal color) to yellow (normal color). (Remark: breastfeeding baby poop's color is yellow.)
The baby diarrhea poop is getting back to the normal color.
The baby diarrhea poop is getting lighter.
How to say the color is changing from darker to lighter?
In what context are you going to say/write this? "Poop" is a very childish way of referring to excrement or faeces. In this context, nothing is required after "diarrhoea" - that is, by definition, very runny excrement.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.