True, I've yet to meet a monster that's green to me and kills me like that. Even yellow monsters are fine sometimes (Dodos in my case, when I began poking them).
It can differ a bit though, I admit that much. I guess certain monster types are naturally stronger a bit, or maybe stronger against certain attacks / elementals or they got stronger skills (these monkeys for example can be a pain even at green). But all in all the colors give a good general idea of how strong that monster might be. So far, at least. When I look at all these huge dragons and ogres and whatever else is running loose...not sure how they will be once they appear as green or blue.
FFXI: Rynaria - Mithra on Kujata [retired]
FFXIV: Fancia Dragonkeeper - Miqo'te on Besaid