I've been having a similar problem with me room mate recently, though not about racism. It's just that I'm so laid back and have such a neutral expression most of the time, that when I do get annoyed he thinks I'm really pissed off. If I ask him if he knows where something I put in the fridge went, or where one of my tools is, he reacts like I'm accusing him of something. I've tried to explain that my facial expression and vocal tone don't always conform to the norm as far as how I'm feeling, but I can't seem to get through.
Incidentally, I used to work a job that through experience would cause even the most well intentioned people to eventually become racial profilers. No, not the police, food service. Everyone knows the stereotypes about black customers and not tipping. Sadly, there is a reason for those stereotypes. I've been working food service for almost 5 years in two totally different parts of the country, and the one constant has been consistently low or no tipping from black identified customers. I use that term, black identified, to refer to people who speak in black vernacular English, and adopt the "urban" mode of dress. My black coworkers had another term for them, that I won't repeat here. They didn't tip them either, so it doesn't seem to be a racist thing on there part, just a general trend. After months and years of the same sort of people stiffing you over and over, you start to lower your expectations whenever you get that sort of customer, and some drivers/waiters lower their level of service in the expectation of not being compensated. This can be misinterpreted as racism on the part of the server, but it is really experience telling them not to break their butt for a customer who is statistically less likely to tip. Living in Seattle, I've been accused of racism many times because I've tried to explain this to people who've never worked food service, it got really irritating. I'd hear the most outlandish justifications for why I was wrong, coming from people who had no idea what they were talking about, uniformly white people. My black friends, would say that's unfortunate, and that black culture does not teach their kids about tipping. Anyway, I've gone a bit off topic here, but I think it's relevant, because service will make anyone into a borderline racist, at least on the clock.