lavender and blue are two different genes. If you want lavender you have to breed to a lavender bird. None of the first generation (F1) will show the gene. If you take F1 and either cross them back on the lavender parent or on each other, you will get either half lavender, half split to lavender babies, or if you breed them together you will get a quarter lavender, three quarters not showing lavender, and two thirds of those non-lavenders will be carriers, one third will be NON carriers, so you will have no way of knowing which is which, so you will have 3/4 "unknown splits" which can still be valuable if you have a lavender to cross them back on.