A sudden change in attitude

Around 6 weeks ago a client brought one of her cats into the clinic for treatment of something quite basic, but a whole new problem arose when she went back home that day. Despite a long relationship, one of the cats decided that he really just didn't like the other cat after all, and raging intercat aggression has ensued ever since. We've tried a number of behavioral techniques too numberous to lay out here, and having no luck, have started reaching for behavioral modification drugs such as prozac. Thankfully, what is going on with these two cats isn't too common, but it's something I have seen before. The last time it happened, the client was the one who stumbled on the solution, and I'll never forget it. There were three cats involved, all having solid black coats. Two of the cats just began attacking the third for days after he had returned from a routine dental cleaning. We eventually realized that the problem was the new patch of white on this cats neck, something the clinic created when we had to shave a small patch of hair to collect a blood sample. The owner thought to fashion a black silk stocking for her cat to wear around his neck, and a harmony was restored.