Biome overrides are a way to replace certain aspects of biomes, that are normally controlled by the server side, with the use of resource packs on the client side, similar to the biomes_client.json in the Bedrock Edition.

They are defined by the biome_overrides.json file in any namespace (directories in assets, like minecraft), having an override effect if you use the same namespace used by another resource pack, and an addition effect otherwise.

{
   "minecraft:swamp": {
      "water_color": "#4c6559",
      "water_fog_color": "#4c6559",
      "fog_color": "#4c6559",
      "sky_color": "#4c6559",
      "water_fog_distance": 15,
      "water_transparency": 1.0
    }
}

• minecraft:swamp -> the desired biome, defined by the biome id, can be default to be applied for all biomes (be careful with this), when there is no biome override for it
• water_color -> the water color used by the biome, can be a RGB color hex or number
• water_fog_color -> the water fog color used by the biome, can be a RGB color hex or number
• fog_color -> the fog color used by the biome, can be a RGB color hex or number
• sky_color -> the sky fog color used by the biome, can be a RGB color hex or number
• water_fog_distance -> the water fog distance used by the biome, can be a decimal number (default:96)
• water_transparency -> the water transparency used by the biome, can be a percentage, i.e. from the scope 0-1