I've created a biome and played around with some of the properties, but as far as I can tell, there are no up-to-date posts on how to change grass/leaf colour. Does anyone know how to do that?

Look at how vanilla biomes do it.

Where can I find vanilla biome classes?

The "Biomes" class. In Eclipse, vanilla source is in Referenced Libraries -> forgeSrc-xx-xxx_mapped_snapshot_xxx.jar

Not sure about IntelliJ

Thanks, there were two things in there that helped me:

 

@OnlyIn(Dist.CLIENT)
public int getFoliageColor() {
    return 10387789;
}

@OnlyIn(Dist.CLIENT)
public int getGrassColor(double posX, double posZ) {
    return 9470285;
}