This quote doesn't really explain it, but does show you are not making it up! Human eye is most sensitive to green which must be in there somewhere.
'An infrequent event can occur where the entire sky appears green. This is called a green thunderstorm. This happens when a thunderstorm occurs at just the right time of day, with just the right height of clouds, and just the right lighting. This can make the entire sky rather than just a thin band appear to be grayish green, and sometimes very green.'