Lavenders should have "Swarthy horn" colored beaks according to my 1988 ABA Standard. I've always figured "horn" colored beaks would be like a typical cow's horn, with pretty much any grayscale color from white to black. I don't know that I've ever heard the word "swarthy" used in a sentence and had to look it up.
My ABA Standard defines...
SWARTHY: Very dark shading over horn or yellow in the beak and/or the shanks and toes.
Of course we are not dealing with yellow, so very dark shading over horn is what we want.
HORN: A term used to describing color of the beak, generally considered to be ivory tinged gray of low saturation and low brilliance.