Strange Pooping Patterns

[Lastfear1]

Hey, I have a 1.5-2 year old European Goldfinch (Male).
Sometimes he will have poop that has mostly white but a little black, I think that's normal.
Other times he will poop greenish, and a little white.
What does this mean?
I don't think he's sick, because he is always playing with his toys, always singing even throughout the molt, and he seems to be a very happy little birdie.
What could be wrong?

[MLaRue]

Do you feed him fresh greens?