Interesting.
Definition of Island Green:
"A dynamic interpretation of our Johnnie Walker Green Label crafted using only Single Malt Whiskies from each of the four corners of Scotland. Johnnie Walker Island Green has all the character of a single malt, but with a richer flavour experience – combining notes of rich toffee, fresh black pepper. The maritime malt whisky Caol Ila provides the greatest influence in this stunning combination of smoke, sweetness and spice."
Definition of Green Label:
"Johnnie Walker Green Label is crafted from a palette of Speyside, Highland, Lowland and Island malts matured for at least 15 years – all perfectly balanced to bring together the intense aromas of crisp cut grass, fresh fruit, wood smoke, deep vanilla, and sandalwood."
Really not sure if there is a real difference or just marketing.