OK, the lavender gene is recessive. That means, in order for a chicken to appear lavender, it needs 2 lavender genes. If you bred a lavender chicken, to a black chicken, the resulting offspring would all carry ONE lavender gene, and one black gene. They would all appear black, and be called "split lavender".
Now, if you bred two of those split lavender chickens together... it would go like this. 25% of the offspring would be black with no lavender gene. 50% would be black with ONE lavender gene (split black) and 25% would be lavender with 2 lavender genes.
Now, if you breed a split black lavender, to a lavender, you will get 50% lavender, and 50% split black.
If you breed lavender to lavender, you get 100% lavender.
Hope this makes some sense. One of the main reasons for breeding black orps into your lavender line, is to work on improving the shape, comb, etc, and making them look more "orpish"
Any questions, please ask.