Wait what happens when their artistic style doesn't supprt black people, or hispanic people? What you do is leave a giant gaping hole for literally the worst type of discrimination. The law protects ALL people and yes hate speech does find its way through the cracks, but it stops people from openly discriminating against huge groups of people.
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).
One can express racist views, but cannot act on those racist views. What you are arguing is someones inability to complete the job in which case it has nothing to do with their desire. If you can't do something physically then it isn't being racist you just lack the ability to do what these people want. This isn't an argument of ability. When does that line of thinking end? Are you harmed when a university doesn't accept you into their college? Are you harmed when you are refused a loan from a bank? Are you harmed when you don't get accepted for a job? I mean there are other universities, other banks, and other jobs right? The problem with your line of thinking is you leave an opening for greater discrimination to the point that it becomes harmful. If you cannot discriminate in one way, you cannot discriminate in anyway.