Actually the thing I found, and I explained this somewhere else, is that level plays a very big roll in your ability to take down mobs.
This is true for solo as well as group mobs. Con color on solo mobs tells you exactly your chances as someone with average weapons, gear and spells. Group encounters, however, are unreliable as far as con color goes. This is where you need to pay attention to level differences.
At level 28 I can solo green ^^ group firerock giants up to level 24. As soon as I try a level 25 I get my *** handed to me.
The thing you have to remember is that these are GROUP mobs. They con green, but that is green if you are in a group. Personally I think it should con dependant upon your current makeup. Group mobs should con higher to a solo player, then con their correct color when in a group. That is just symantics though.
The same thing can be seen on the opposite end of the spectrum. If a mob cons grey to you, by only 1 or 2 levels, they will still be able to hit you semi-frequently. However, when you hit the magic threshhold of level difference they rarely hit you the entire encounter.
Group mobs will always be stronger than what color they con to you as a solo player. Keep that in mind and judge the encounter based off of level and you should be fine. Just to reitterate, levels are always more important than your AC. AC is more of a way to compare your effectiveness in combat when dealing with things situationally - if that makes sense.