It turns out the Stone colour is not a true grey but a greige, which is probably why it doesn't work so well with Cosmic Orange.
The Marigold Yellow mobee leather case I originally purchased probably would have been a good match, but as mentioned I returned it because of its Camera Control cutout design and because of the metal camera cutout ring (which could block wireless signals and scratch the plateau).
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I'm thinking a dark blue case would also work well, although that gives some Minions vibes.
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I'm also hopeful that some new non-Apple lanyard silicone cases appear soon. Most of the current third party ones out there don't seem to be designed that well IMO.