allBiomes = forAllBiomes()
desert = forBiomes(2)
frozenRiver = forBiomes(11)
allBiomes.set(genVillages, false);
desert.removeAllSpawns("MONSTER")
desert.addSpawn("net.minecraft.entity.monster.EntityBlaze", "MONSTER", 100, 4, 8)
desert.addSpawn("net.minecraft.entity.monster.EntityMagmaCube", "MONSTER", 50, 4, 12)
frozenriver.addSpawn("net.minecraft.entity.monster.EntityGhasts", "MONSTER", 100, 4, 8)

Why does this code nothing?

That can cause a lot of problem I'm not going to fix. Run it in a plain Forge setup and verify the issues persist.

You're going to have to be more detailed. Provide the config file, logs, etc.

"[16:55:02] [Server thread/WARN]: Mod BiomeTweakerCore has been disabled through configuration" in my log file. Why?

That happens. Not an issue. The "core mod" doesn't actually do anything so it gets disabled. The actual "mod" is still running.

I added this string:
allBiomes.set("grassColor", 304864)
But the colour of grass is the same.

[17:56:19] [Server thread/INFO]: Registered command BTReload with permission node me.superckl.biometweaker.server.command.CommandReload
[17:56:19] [Server thread/INFO]: Registered command BTInfo with permission node me.superckl.biometweaker.server.command.CommandInfo
[17:56:19] [Server thread/INFO]: Registered command BTOutput with permission node me.superckl.biometweaker.server.command.CommandOutput
[17:56:19] [Server thread/INFO]: Registered command BTListBiomes with permission node me.superckl.biometweaker.server.command.CommandListBiomes
[17:56:19] [Server thread/INFO]: Registered command BTSetBiome with permission node me.superckl.biometweaker.server.command.CommandSetBiome

Are you running Cauldron?

Yes.