It's very nice
I find the hero bar and the elite bar should be inverted.
The contrast between the gold and the purple tells that it's something very very important. First point to be a Hero.
Now, about the colour of the health bar itself : the yellow bar instead of the red, for the elite, tells that the monster is more experienced than the others. But the purple tells that it's very serious now (especially a dark purple like this).
I don't have made any colour kind of studies ^^ but that's how I see it
My progress trough Tiers of monsters was: Iron > Bronze > Silver > Gold. Might just remove the iron part.
I would say better Gold > Silver > Bronze > Iron, as the gold is more precious than Silver, which is more precious than Bronze, and so on. In Minecraft, the fact that the Golden Sword was worst than the Stone Sword has always troubled me..