my little brother (6 years old) went to the bathroom, but his poop was green. Now, this was a very light green, like St. Patrick's Day green. Is there an explanation for this? Is he sick? Or something he ate?

It could be something he ate,but if it continues,he needs to see a Doctor.

Green poop can be a sign of gallstones, He needs to get checked by a doctor.

So far he hasn't pooped green again so we'll see

Liverock,
Gallstones in a 6 Year old Child???

Certainly children can get gallstones, although its not as common as with adults and can be due to hereditary. If you dont believe me just google children+gallstones.

If he is not producing green stools regularly then it is more likely to be due to 'a rapid transit occurance'. Green bile is stored in the small intestine and when the stool is formed it slowly changes color from green to brown whilst moving through the large intestine before being eliminated.
If you get too quick an elimination then it doesnt have time change color.

purple koolaid will make green poop...Applejacks cereal use to have a green dye in it that would also do it... Green candy, green drinks of any sort.... the amount of food dyes in food I have seen a variety of different colors.... especially after eating birthday cake with colored frostings!!! My oscar the grouch cake made for some interesting colors!!!!

__________________
God is and all is well
~John Greenleaf Whittier~