Green and gold are Australia's national colors. From this
Australia's national anthem, Advance Australia Fair, is a Revised Version of a late 19th-century patriotic song. It was declared the national anthem in April 1984, replacing God Save the Queen, which was designated the royal anthem. In the same year, Australia officially adopted green and gold as its national colors.
I wondered the same thing during the opening ceremony. Now if someone knows why people were wearing what looked like green and gold socks on their hands at said ceremony, I'd love to know.