I was just guessing with the egg color. But if it is a mixed breed I don't think the color of egg it hatches from makes a difference. If it is a pure Black leghorn then it will lay white eggs. If its a mix you will have to wait and see what color egg she lays.
If it's a mix with brown egg layer I would guess it will lay brown eggs, how dark of brown will depend on how dark of egg one of the parents lays, if the other parent is a white egg layer it will dilute the brown some.
I have Lavender Orpingtons. The chicks are grey with very light yellow on the chest and belly. I also agree it is not lavender or Marans. Looks like possibly a mix.