Teaching toddlers about equality

So my son is 3. We have always taught him to treat people with respect, but we have never directly addressed races of people. We always just treat everyone the same and we're hoping he would grow up to be the same way. Kids aren't born discriminatory right? Well my little boy just told me his white fire man toy was a "sun" man and the black fire man toy was a "dark" man and that he doesn't like the dark man because that's scary. I have no idea where he got this from. I explained to him that no matter what color someone's skin is, we love everyone and treat everyone with kindness and respect. Just because people look differently doesn't mean that they are scary or bad, because being different is beautiful. He argued with me saying he doesn't like when people have dark skin. I repeated my points and just told him we don't need to think about people that way because everyone should be loved even if they look different than we do. I'm at a loss. How do I teach him differently? Any book recommendations to read with him? Any ideas on how to teach equality? I know he doesn't understand, but this isn't the way I am raising him. I want to fix this.