The mod is using the built-in function that calculates the color for the durability bar, I might see about copying that into my mod and allowing a config to set the gradient range.

(I'll look into adding this when I create the 1.15 version, just waiting for a stable release of forge 1.15)

I can't easily add support for other mods, this mod just reads the item durability, if the other mods aren't using that (which they should be) to keep track of durability, it would be a lot of duplicated and mod-specific code to add them, tinkers does but they multiply it by 10 which adds an extra 0 to the number.

Since this is a coremod it can't use forge's built-in mod communication, it would have to do nbt or metadata reading for every mods format.

To any mod developers out there, please keep track of tool/weapon/armor/etc durability with the actual durability instead of metadata or nbt, durability was made for this exact purpose, don't recreate it needlessly.

I found Minecraft uses a built-in hsv function to select a point across a gradient, I can change the start and stop points but not multiple points across the gradient, and since Forge coremods don't have a way to allow configs, it would only be configurable on Fabric and Forge 1.12

I know it's an old issue but couldn't you use your own calculation for custom color? I did a quick snippet of the code and that would be more then doable