There are blue brahmas. Maybe breed the blue to a light so they will be a more lavender color and boom you got a lavender brahma? It will take lots of breeding a keeping back on the breeding stock your going to want.
First you need some kind of lavender to enfuence the lavender gene, so like lavender Orpington. Im not sure how many brahma colors there are. Your going to need some kind of brahma color that will work good with what your wanting.