Interesting. Isn't this a loophole? If I am the baker in question, can I refuse to bake the cake citing the shirt color instead of the real reason, which is that I am homophobic? (I am not, just asking a hypothetical question)
If you kick everyone out of your store that wears dreadlocks, no matter what their race or protected class, and that has a disparate impact on people who belong to protected classes, that's also illegal.
Similarly, if you refuse service to, say, immigrants because of the color of their shirts, but you serve everyone else no matter what color their shirts are, that's also illegal.
That might be difficult, especially if you were lying.