Is mixin.features.fast_biome_colors completely using fast_biome_colors?
Fast biome colors improves the performance, and I just want to make sure fbc is fully implemented.

Force-disabling mixin 'features.fast_biome_colors.MixinBackgroundRenderer' as rule 'mixin.features.fast_biome_colors' (added by mods [colormatic]) disables it and children
Force-disabling mixin 'features.fast_biome_colors.MixinClientWorld' as rule 'mixin.features.fast_biome_colors' (added by mods [colormatic]) disables it and children

Colormatic currently requires vanilla colormap handling to be present so it can inject into it. Sodium also does this which is why we disable it in Colormatic. In the future this restriction may no longer be necessary but I wouldn't hold your breath.