Yay! I finally get to be useful. I tracked down this addon:
http://www.tukui.org/addons/index.php?act=view&id=274, for Elvui a few weeks ago for exactly the same reason.
This will allow you to set a breakpoint at 40% (Turns the healthbar to selected color) It has an upper and lower breakpoint, I just turned the upper one off. Set the lower 40%, with a brightred color. Works perfectly.
Just drop the addon in your folder, and modify "Fixed Health Colors" under unitframes in Elvui.
I tried to just watch health percentages, but found it too tedious and I don't like unnecessary stuff on my health bars. On raid fights where you're running echoes, with torrent I've found some use for this legendary.
I have pretty mixed feelings about this item though. But if that's what you got, make the best of it.