Yeah it works on Optifine, so it's behaving differently.

Please provide a description. For example, specific datapacks or mods for which Colormatic does not tint biomes.

Install https://www.planetminecraft.com/texture-pack/incendium-s-optionnal-resourcepack-remix/ and incendium (https://www.planetminecraft.com/data-pack/incendium-nether-expansion/), and even after changing the lava colormap to apply to a solid block instead (so you don't run into #62), you'll notice the proper colors do not apply in any of the datapack's custom biomes, instead resulting in an orange tint.

This is because IORR uses the grid format for its custom lava colors. There is no standard assignment of grid columns to custom biomes (to my knowledge - let me know if Optifine is different), so Colormatic just falls back to the default color.

If Optifine behaves differently, then this would be a parity bug.

(This still has the awaiting response tag, by the way)