Is the green you are seeing a very bright neon green looking something like this?
In which case it could be biliverdinuria
. This is typically an indication of internal organ failure either due to infection or inflammation of usually the liver. Feel the keel of the bird. If it feels narrow on the bone without much mass, the bird is likely starving.
Most common cause of liver failure in a pigeon or wild bird is hepatic lipidosis. This is caused by over eating seed and carbohydrates such as bread and other grains. Once the liver is full of fat, it can not function and begins to fail. Bile increases in the blood and ends up in the droppings.
"Birds with fatty liver disease must be placed on a low-fat diet, which includes high quality pellets and fresh fruits and vegetables. It is sometimes difficult to get them to eat, so tube feeding may be necessary during the beginning of the treatment. They usually need additional supportive care such as supplemental heat and fluids. Additional medications, such as lactulose, may be given to treat or prevent central nervous system signs."
Darker greens can be a sign of bacterial overgrowth or even coccidial parasites. These are not as common. For bacterial overgrowth, an antibiotic is usually needed. Amoxicillin has been used in birds. For coccida, a medicaion called Albon is often used.
A simple fecal test at the Vet can tell you which medication to use. This is a very cheap test, and the Vet may not even require to see the bird, only the droppings. This might save you time and stress in the future when you see other birds with similar signs.
Hope this information was helpful to you,
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