The fix is a bit fussy.  Click on the Paint-bucket tool in the editing tools.
Then click several times slowly on the background.  The first click doesn't seem
to do anything, but sooner or later you'll get a color-picker, and you can
get the proper color back.

I don't know why a single click doesn't do it, but for some reason it's
a slow response.  (Edit after another trial: You might try clicking on the
outermost frame of the Poser app before clicking in the background.)

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