Why is "In general, it is green." not accepted?
It should be. Report it.
"it is green usually" not working
Then report it.
"It's green generally" was not accepted. I think it is not the case when the words order matters.
The most common usage is probably **It is generally green."
In English, there is usually an attempt to have the adverb near the verb. With the exception of some very flexible adverbs, adverb placement follows specific rules in standard English.