Black: When I first had Black, I was just getting into whiskey. I had tried Red and thought it was horrendous. I had tried Glenlivet and disliked it as well. Black I thought was very tolerable at first, but had a "smoky" flavor. I went from Black to Blue. After I started trying some other whiskies, I went back to Black and discovered it was actually quite good. I believe it has a lower alcohol content than Johnnie Green, but the flavor is better. In terms of personal choice I'd rate Black above Green.
Green: It has a more burning sensation going down. Probably to be attributed to it's higher alcohol content. It's still a good whiskey, but I prefer black. I would tell you to try Gold because Gold is a LOT better than Green, and about the same in terms of price range.
I need to try Yellow: Johnnie Walker Swing
Blue is great, but at 160 a bottle - some places 230, it's a rip off. Plus I drink fast, so I even though I enjoy it, it just isn't worth it for me.