The durability bar fades from green to red using a linear fade in the RGB colorspace. This results in a horrific 50% brown color for 50% durability, which both looks bad, and suffers in terms of visibility. The entire point of this bar is to convey information to the player, having to squint to even see it is a disappointing shortcoming in terms of UI design.

Instead, I propose fading in the HSV colorspace, like we know how color works, and pinning saturation and value to 100% and spinning the hue knob, resulting in the colors in the 2nd, 3rd, and 4th row of the attached image.

Related: the third and fourth rows of the attached image also do away with the weird pointless second box drawn in the damage bar which further obscures the data the bar is meant to convey.

Also related: Everyone I've asked prefers the 3rd row (with the thin bar) over the 4th row (with the double-thick bar), but nobody can tell me why.

In any case, if you need code to transform colorspace from HSV to RGB, you can google it, but there are more efficient ways to do it for this simple (and singular) application. A lookup table, for example. It would only be 256 (or 13) entries (depending on choices made). Alternatively, you could just let us define the bar graphics in a resource pack, with a new 256x16 (both 2^N) texture.

Here's to being able to read durability bars in the future.