The "computer-style gradient" graphs will give you what's shown in my screenshot. It's not exactly what you want, but similar. (With the Windows Aero theme, blue until the bar reaches 90%, then red for higher.)
Hmm, I have tried that for a day. And most of my drives are around 85% to 95% filled, leading to some drives appear blue and other red. That's some kind of confusing for me.
Maybe it would be possible to introduce another augmented gradient mode as improvement that fills the bar with a solid color from C1 to C2?