ATC used to be able to get a red ASIC if they had an operational need as a private pilot, even if they were (for example) working in one of the centres and otherwise eligible for a grey ASIC. Five or so years ago it was decreed that we couldn't do that any more - the operational need had to be a work-related one - so there are a fair number of ATCs now with a grey Airservics ASIC and a red AUS one.