I'm really sorry for your situation. Maybe you could demand that your parents read or watch a bunch of books/movies that would better educate them on the subject? Books on racism, discrimination, the history of people of color in the US, etc. and tell them that until they've done their "homework," you guys won't be visiting them? You could put together some kind of syllabus, so that their "effort" becomes a more legitimate apology. No idea how to verify that they worked through it but they could always email you weekly summaries of their thoughts on the material for a few months. Working through a few books and movies could easily take six months of time, enough for your emotions and those of your husband to also settle down somewhat (perhaps) and perhaps making your parents more sensitive about their words.