This can be used to generate a datapack custom biome colours that you can apply to your world.

This tool does not automatically add biomes to your world and only sets the colours, not the full biome settings! You must use WorldEdit or a similar tool to apply the biomes to an area. After that, the biomes should appear in vanilla without the mod enabled (Datapack still needs to be there).

Usage:

Run the program and make the biomes you want.
Open the output folder.
Select data and pack.mcmeta.
Right-click and compress them into a zip archive.
Place this zip file in saves/YourWorldName/datapacks.
Apply the biome to an area with another tool such as WorldEdit.

WorldEdit Example Command:

/br biome sphere biome_colours:<name of json>

You may have to leave and rejoin the world to see the changes.

Github