Australian $100 note with blue ink portaits
In 1996 Australia introduced the New Note Series $100 note featuring Dame Nellie Melba on the front and General Sir John Monash on the back. On a small number of 1996-dated Fraser/Evans $100 notes, the portaits of both Nellie Melba and John Monash are printed in blue ink rather than the usual black ink.
The circumstances around this variety are not known - the blue ink is similar in colour to that of the New Notes Series $10 note portraits, but that may be coincidental. There is at least one known example, whose with serial number BX 96 865 733. The BX serial number denotes that it was printed with test ink, but the portrait colour was not the subject of the ink testing. Given that this is currently the only known example, it may be that only notes on the same sheet had the same blue portrait.
An example was made available for sale by Strand Coins in around 2008 and was still available in 2012, though it is unknown if or when it eventually sold, or for what price.