"The only downfall I see to having severs which would probably kill the entire idea of having colors for severs would be the idea of mobs with multiple limbs. Most of the time they would only have a very limited amount of limbs to sever. It would not turn the entire combat dialog in to a cross dressing rainbow fest;"

Even if a mob has no limbs left, you can still get severing crits AFAIK. And furthermore, how long do you need to kill the average mob..? Have you ever been in the canyon?