Male and female plumage:
The lavender is not a lighter shade of the blue Pekin. It is different genetically and is of a lighter more silver tint without the darker shade associated with the normal blue. The silver that is most obvious in the neck and saddle hackle feathers of the male.
Lavender is a recessive colour gene meaning that a bird can be carrying the colour but not showing it because to have the lavender colouration a bird must have two sets of the gene, one from the father and one from the mother.
This means if you mate a pure lavender bird to a pure black bird, the chicks will only be black but if you mate a chick back to its lavender parent you will in theory get 50% lavender or 2 chicks out of every 4.