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I also start to think there might be no cases at all. Which would make at least a little sense for the red, yellow, blue, and red as those are the colors people are supposed to show off. But what about the black and white??
Even the for the colors, they could offer matching silicone cases. You can actually find some of the colors available for the XS and XS Max.

There are matching Beats lighting earbuds for order but they must have sold out because last I saw, colorful ones weren’t shipping until November. Black was available to ship.

But the earbuds were the only color matching items for the XR that I saw.

I’m wondering if they had a problem with the cases yellowing. Otterbox clear cases for the X and XS went completely dandelion yellow within two weeks according to Amazon reviewers. They’d actually look perfect for the yellow XR.
 
It's likely that they meant for the XR to be used without a case because they went to great lengths to release so many interesting colors. I wouldn't be surprised if they had a strong conviction along those lines.
 
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