Rusty, my dog that was a stray for about 3 yrs freaks out if someone is wearing red. He's ok with me wearing it, but he trusts me completely. I first discovered it when my mom was visiting and she put on a red coat. He had been fine with her until then, but then he charged her barking and growling, to the point where I had to step between them. She removed coat, and he was fine. She tried it again a few days later and same reaction. Then, a year later, after she had been visiting for a week, and he had been fine, she came out of the bedroom wearing burgandy-red sweat pants. He started slinking around with his tail down, eyeing her suspisciously. She changed he pants and he was fine again. I know when he was a stray, people had tried to catch him and were mean to him (throwing rocks at him, one guy intentionally drove his truck off the road trying to run him over (I saw him do it and confronted him...he said he was tired of him sitting outside his fence making his dogs bark). So I wonder if someone wearing red did something to scare him, or tried to throw a red blanket over him.
Could your dog have had a bad experience with someone wearing black and white checks?