The Australian standard dictionary is Macquarie, which gives both pronunciations (in phonemic symbols, which I shan't try to do here), and describes it as a dark brownish red.

Interestingly, Macquarie gives two entries for Maroon, indicating that it's two different words in origin even though they are now spelt (and usually pronounced) the same. It includes the firework along with the colour, and in a separate entry (where only the `oon' pronunciation is given) it includes being cast away and the escaped slaves.

It seems as if the Australian pronunciation has etymology and history on its side.