@The Moonshiner is correct, you cannot create lavender. If you mean geese, either buy lavender geese (probably easier) or breed two blue geese (like Steinbachers) together. 25 percent of them will be lavender, the goose equivalent of splash. It breeds true. 50 percent will be blue and 25 percent will be grey.
If you are speaking of “silver” the duck equivalent of splash, the same thing goes. I don’t know if you want bibs, but if you do, use blue Swedish, or buy silver Swedish. Or use blue-bibbed Calls.
If you do not want them, the only breed with ready-available non-bibbed blues are Runners, known as Cumberland Blue Runners.
Or, you can use a “recessive blue” Mallard or Call, which is the duck equivalent of lavender. Unfortunately, these are extremely rare, and the only ones I’ve seen are mallard-based.