You need to start with a lavender bird. Lavender is a recessive gene, which means it doesn't show in the first offspring. The lav bird in my avatar is Lester as a youngster when we started the project. He was bred to black orpington hens, which resulted in black chicks carrying the lav gene. Those siblings were then crossed and some of the daughters bred back to Lester to produce full lavender birds. Once you breed lav to lav, you will have self breeding lavender, so you won't get black anymore. Lavender is also known as a self blue, which means it breed true to color once you have rooster and hens in that color.
You can read about the project and see some of the first pics of the birds we started with down towards the bottom of the lav orp project page.