Then you're a racist artist, and so be it. It's not illegal to be racist or hold racist views: just look at all those abhorrent white supremacy groups.

It's not like someone is actively harmed because you only paint pictures of black people. Maybe you only like painting with dark pigment. Maybe you feel it's a specific skill you have and your work painting East Asian or Caucasian people would be poor and harm your artistic brand. Or maybe you actively dislike white people. Who's to know?

(For what it's worth, I used to do amateur portraits and vastly preferred sketching old men. The crags, the wrinkles, etc. I found them much easier and more interesting to do. Had I decided to charge, I almost certainly would have refused requests from younger people, as I wouldn't feel they would be happy with the result. Is that discrimination, given it's based on age?)

Either way: people need to consider if harm is really being done to the person discriminated against. Does it "harm" you if a particular artist won't make you the product you want? No. Are you harmed by a particular business not making you a cake? No. And if they had to turn your business away because they were too busy, you wouldn't have a case.

In future, bigoted businesses will simply lie and say they have no capacity to fulfil your order.

The worst kind of discrimination is being refused a job because of your [group]. It's being harassed or bullied. It's being killed.

It's not not-having-someone-paint-your-portrait (or not-bake you a cake).