If they are true lavs, then if you breed lav to lav, you get pure lavs. The lavender gene is a recessive gene and the birds must have two copies to show it. Birds with only one copy carry the gene, but dont show it, ( they are solid black)
So if you bred a lav (2 copies ) to a black ( 0 copies) all of the first generation chicks will be Split lavender (solid black colored but willl carry one copy of the lav gene.)
So you can either breed those offspring together and that will give 25% lavs, 50% Split lavs, and 25% blacks.
Or you can breed those first generation chicks back to the lav parents and get 50% lavs and 50% splits.
And like I said eariler, Lav to Lav gives 100% lavs.