We've already addressed why your finger might be turning green, so why not investigate a little further downstream? Well, thankfully, Hank Green and the excellent SciShow are here to help us. In their most recent video, they tell us why our poop might be green every once in a while.
Normally, poo is any number of shades of brown. But, the video explains, that's all because of one chemical: the pigment stercobilin. It's created along the digestive tract when your gut bacteria metabolize the yellowish-green bilirubin in bile. The biproduct formed here reacts with oxygen to create stercobilin. So, if your number-twos look like you swallowed your lawn, that might be because they're traveling through you too quickly — before your gut microbes have time to eat their way through all that bile.
There are plenty of mostly-harmless reasons why this might be happening (maybe you're a bit hungover or getting over some food poisoning). Or, even if everything's happening at the right speed, you still might be going green if you just ate a bunch of food with green dye in it — or blue and yellow dyes together. (Yep, the rules of kindergarten color-mixing hold true in poo, too.) Of course, if you're pooping pea-colored hues every day, go see your doc. But, if it's a one-time phenomenon, there's no reason to stress. Who wants to wake up and see boring-old brown in the toilet every day, anyway? A little variety is good in life.