After installing Colormatic, everything appears to work; though in biomes with custom coloring for grass and leave blocks, it is automatically reverting to green regardless of the biome (instead of in my case it being purple in my Skylands biome).

Please provide your resource pack.

I am currently using New Default+, Tissou's Zombie Pack, and Better botanicals and vegetation.

Do you have a datapack as well?

I'm not 100% sure. I am using Better Minecraft for 1.18.1 modpack. Couldn't find a datapack folder in my install and the datapack folder on the server is empty.

Do you have a Colormatic colormap defined for grass and leaves? Colormatic will apply custom colormaps to all biomes, even custom biomes. It tries to assign colors realistically, but when biomes have non-vanilla tinting there's not much that can be done.