About all you could do is gather as much information as you can and let them know. If they are in it for the money they wont care and if they are in it for bettering the breed they will listen. But, like Dan said there is no legality that someone can not breed a white boxer.
Me personally - Nothing you can do. However if my dog's breeder was using a white, and my dog is carrying her kennel name, I would IMO be extremely embarrased. So I may send an e mail or letter that may fall on deaf ears but hey, to me it would make me feel better about the situation. I would just say that being a boxer owner, and an owner of one of your dogs I have learned many of things since being that owner. One is that its unethically to be using a white boxers as a stud, such as you are and wondering the reason behind it.
They may not even know any better, or not give a rats butt. However in a 50/50 chance, giving them info and telling them that their kennel reputation is on the line for using a white, may in fact make them want to make adjustments. Or your letter / e mail may find its way into the trash - but a gamble thats not going to cost you anything, but could make some difference shall they listen.
***From a dinner with my dogs breeder, she openly admitted her stories of starting off as a breeder over 10 years ago. She said her first litter she bred a white dog. Then only to come to find out it was not allowed and she almost freaked because she did not want to be known as someone who was breeding whites. To gain respect in the boxer world is to do things the right way, and the ethical way. Those who don't care, are not respected and someday, I hope if I ever get into breeding, to be a breeder who has respect from the Boxer Community.