I wondered.. why not make it completely inverted.. with the red bars in the screenshot black and the background going from green (100% HP) to yellow (50%) to red (0%)?
Some way you want to see the end of the bar though.. so Grey maybe is better than black.. or some border around the black bar..
On the target health I (as melee) mostly targets mobs so there maybe the coloring should be reversed.. (red = 100&, green = 0%)
Otherwise a very nice idea.. and I look forward to the result..