Hi.
Thanks for your query.
Noted the history about 18 month old having pale yellow or pale green stools once in a while,
constipation, drinks a lot of milk, wants to know the reasons as google scared you.
First of all stop searching on Google as there is no interaction and the answers will be standard and not differentiated.
The cause of the colored stool is not the milk but can be due to the following reason:
Intestinal infection.
Imbalance of the bacterial flora as there is adjustment to new exposure.
Half digestion occasionally.
I would advise you the following:
Send the sample of stool for tests including for intolerance.
Give probiotics.
Antibiotics only if there is an evidence of infection.
Try to make the milk containers, bottle sterile.
Take care.