[1.7.2] [Biomes] editing deserts top few layers blocks?

Hey!

So Ive made a few new shades of sand blocks and have them in game working, id like to edit the way that deserts generate and add patches of my different shaded sand blocks to it.

I have noticed that in the BiomeGenDesert.class it has the following:

this.topBlock = Blocks.sand;

this.fillerBlock = Blocks.sand;

I thought it had something to do with that? but im probably wrong and even if it is I dont know how to set that from my mod class with more than 1 block.

Any help?

Thanks!