Lav Roo x Barred Hen
Offspring will be:
Pullets - Black, no barring w/ one copy of Lav
Cockerels - Black, Single copy of barring gene (dark barred), w/ one copy of Lav
Crossing of offspring will result in different Phenotypes
Lavender Pullets and Cockerels - No Barring
Split Lavender Pullets and cockerels - No Barring (appear black)
Lavender Pullet and Cockerels - With one copy of barred gene, pullet is finished, cockerel needs second copy to be finished.
Split Lavender pullets and cockerels - With one copy of barring gene (appear black)
Breed the Lavender Barred birds to each other, cull the resulting single barred cockerels, keeping only those with two doses, and bingo, barred lavenders breeding true for color.
I am sure we have completely lost conformation along the way.
Good luck