The story is told on this 15 minute video describing Brand Building the Orange way.
It describes some unconventional gut-based decisions that were highly innovative at the time.
The idea was to create an optimistic, communal, aesthetically beautiful brand that people just wanted to be a part of. It was a wildly radical move at the time and has been widely copied since - think telus.
The presenters are both honest and self-serving, as it is part of an upcoming series from the UK-based IPA whose mission is to promote the value of communication agencies. So take it with a grain of salt but take it.