A quick Wiki search shows the difference between all of the blends of Johnny, very interesting:
Red & Cola – a premix of Red Label and cola, sold in cans and beer-bottle like bottles. This product has also been marketed under the names "Premix" and "One".
Red Label — a blend of around 35 grain and malt whiskies. It is intended for making mixed drinks.[4] 80 proof. 40% ABV.
Black Label — a blend of as many as 40 whiskies, each aged at least 12 years. According to William Manchester, this was the favourite Scotch of Winston Churchill.[5] 80 proof. 40% ABV.
Green Label — a vatted malt whisky that consists of a blend of about 15 individual single malts, the signature malts being Talisker, Cragganmore, Linkwood, and Caol Ila – Aged 15 years. 86 proof. 43% ABV. Previously sold under the name 'Pure Malt'.
Gold Label — a rare blend of over 15 single malts, including the very rare Clynelish malt. It was derived from Alexander II's blending notes for a whisky to commemorate Johnnie Walker's centenary[citation needed]. His original efforts were thwarted by a shortage of these malts following World War I. Gold Label is commonly bottled at 15 or 18 years. 80 proof. 40% ABV.
Blue Label — Johnnie Walker's premium blend. Every bottle is serial numbered and sold in a silk-lined box, accompanied by a certificate of authenticity. There is no age declaration for Blue Label (Although it states on an information booklet, provided in the box, that some of the blends used are "up to 60 years old"). 86 proof.
It appears that the different colors of Johnny doesn't mean that one is stronger or smoother than the other, possibly like a VO vs. VSOP or even an XO of a cognac.
Johnny is a Scotch/Whiskey Blend and a quick online search shows that "Green" is about $20 more than "Black" and "Gold" is about $20 more than "Green", of course "Blue" is out of the box hovering around $200+.