I know the white color stands for peace in Islam but I don't know why the green color is used in so many Muslim flags.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and  Messenger.

There is no particular significance for the white or green colours in Islam. In fact, if there is any particular significance to any colour for Muslims, it is a conventional one not a legal one. So, the person who does not comply with this indications is not against the Shari‘a.

Allah knows best.