Why are Australia's national colors green and gold?

Australia's National colours are not blue and gold. Blue and gold are Australia's "heraldic colours", being the colours of the wreath in Australia's coat of arms. Further, blue and gold were selected as the colours of the ribbon of the Order of Australia in 1975.

Australia's official National (and unofficial sporting) colours are Green and Gold, the colours of our national emblem, the Golden Wattle. These were established in a proclamation by Sir Ninian Stephen, the Governor-General of Australia on 19 April 1984.