I think we are in the same predicament here, both theists and atheists alike. At a practical level we have to try to appeal to shared values and similar moral intuitions, that is all we can do. Some times all it takes is for a racist to actually personally get to know someone from the “outgroup”, so they can experience in their own life that all the demonization is either both unjustified(and thus unfair), or that it affects people who are essentially just like themselves and capable of suffering.
Esoteric philosophical arguments about what should be the moral standard, and whether this is truly objective, aren’t very likely to change anyone’s views on these things. To persuade the racist out of racism, you have to appeal to their (hopefully existing) empathy. How would you like it, if it was done to you? You have to find out whether the person you’re talking to have a sense of fairness and is capable of feeling any sort of empathy for others. That’s in the end what is most likely to make someone “tick”.
I also think there’s a very significant difference between someone who has unconsidered or unexplored racist biases(which we probably all do to greater or lesser degrees), and someone who is a committed ideological racist.
In my experience, ideologically committed racists are almost impossible to talk to. And I mean that quite literally. It’s rarely possible to even get a real discussion started because no matter what you say they have some completely hermetically sealed network of conspiratorial rationalizations built up that provides an “answer” to basically anything you say(the fallback is, of course, always that it’s “the jews” who are orchestrating the collapse of civilization). It’s like talking to flat Earthers, where any putative evidence or argument against their position is dismissed as “these satellite pictures were photoshopped in a NASA bunker”. With people like this, conversation is utterly fruitless. It takes some sort of radical lived experience to change their minds, and you can’t do that through mere casual conversation.