There are two other issues that might cause the discoloration. The first might be that your birds came from someone that was crossing Black Tailed Whites with Black Tailed Buffs in order to either improve Black color in the in the Buffs or improve the type in the Buffs. The other issue is possibly a brass color that crops up in some strains of Black Tailed Whites. The brass is strongest in the male wing bows and saddles. I need to see the color to tell which it might be. It all depends on what you intend to do with the Japs that will determine your recourse. If you want pets then there is nothing wrong with them, but if you want to show, either is a problem.
Terry