I wish for the OP's sake that they had included examples of non-white people they asked to attend (but who apparently couldn't make it). It'd at least give a little more insight to who they think is a good panelist. But turning the detractors' argument from "Hey, there's just white guys on this panel!" to "Hey, please include a minority so that no one's feelings are hurt" was probably the least useful way to counter the detractors.