A true "lavender" is a black bird with lavender added.
How to get one depends on what you have to work with. Asumming you have the genes wanted between two birds you breed the birds together to put those genes into one bird then you continue breeding those birds until you breed out all unwanted...
Doesn't work that way. Can't make blue turn into lavender. Sure you can focus on breeding the lighter shades of blue but they're still gonna be blue and won't breed true like lavender.
There are true lavender brahmas.