Why did my 1 year old poop really green?

There are many: reasons for this, but mostly that everything went through his system quickly. Did he/she eat or drink anything green? Is there fever, vomiting, new foods, travel? Is the stool loose or watery? Any blood? I would recommend calling your child's pediatrician with all of these questions answered and they should be able to help your figure out the cause and tell you if any treatment is necessary.

Poop color : Most likely due to something he ate or drank

Bile: The greenish color in stool is the breakdown product of the bile when it has not been in the gut long enough to go through a change to yellowish or brown. It is considered normal Some digested material will take on a darker tint than others & the germs in the gut will also impact the final product.It could be food coloring also, but my bet is normal bile stain. I'd ignore the whole issue.