I like this post, but it feels like it's missing something to motivate the clever title. You didn't seem to identify a "grue" (presumably, some horrible consequence from not being luminous), which doesn't seem terribly difficult to do in this case. Perhaps it doesn't belong in this post, but surely the title demands one.

Also, is "luminous" a modifier for beliefs, people, or both? Should there be two different words? Did I miss that discussion along the way?