The treasury tags next to my keyboard are definitely green. The box also states that they're 51mm long and this seems to be a standard measurement.
I've always assumed that 'treasury tags' are so named because they were first ordered by HM Treasury. It would have been their civil servants that determined that they should be 2" (=51mm) long and coloured green.
I have nice orange ones with white plastic ends.Being a civil servant we have loads and loads of them but with the advent of the paperless office, they will become a rarity.
People will be selling them on ebay soon so hold onto your boring green ones!!!