It can be caused by a lot of green vegetables like spinach but it does not sound like you have eaten enough for that to be the cause.
More likely is that the green colour is caused by bile. Bile starts as a yellow/green colour and becomes brown in the colon as the bacteria there help to break it down. If the contents of the gut move too fast or the bacteria as disrupted the green or yellow bile ends up in the stool unchanged. Green stool and diarrhea can be a symptom of salmonella infection for this reason. Also babies commonly have green stools as they lack gut bacteria to digest the bile.
Interestingly a average adult releases about 1 litre of bile every day for digestion. Most of this is normally reabsorbed by the body.
I would speculate that if you suddenly start eating lots of fat and little fibre you produce more bile and the bacteria in your colon will be disrupted for a while. A dose of bile may have just gone straight through you.
It will have nothing to do with remnants of your prior diet.
If you are worried about it, symptoms continue or you get any other symptoms go and see a doctor. As well as asking random people on the internet.