It seems the use of the phrase "Fanta pants" reached an all time high in 2008, enough for it to be added to the Macquarie online dictionary. The popularity of the "crude nickname for redheads" is part of an overall global rise in prejudice against ginger haired people.
Macquarie is still accepting votes for the 2008 word of the year. You can find Fanta pants in the Colloquial section.
// (say 'fantuh pants)
noun Originally British Colloquial a person whose hair is naturally red. [from the orange-coloured soft drink Fanta + pants, with reference to pubic hair as the indicator of hair colour]