That is a really good genetics question. Since my understanding of recessive white is that it covers all colors (it represses any pigment in the feathers) - it would cover the black that lav would would modify - so it would be hidden. It probably shows in the chick down (the gray) but would disappear when they are adults. It is possible the chicks that are mostly gray are actually lavender, and do not have two recessive white genes.
Has anybody else tried this mix in real life and what were the results?
This is exactly what I am wondering

