Changing default background color to #0C0C0C causes a different shade of black to appear in areas where there is text or whitespace. Bug or not?

[zch]

If I change the default background color to #0C0C0C I get a different
shade of black background where there is text or whitespace:
https://i.imgur.com/B7XIHCs.png

This does not continue if I change the background to, say, red:
https://i.imgur.com/1S0nNHC.png

Even with a fresh emacs session, ``emacs -Q'' does not alleviate the
issue https://i.imgur.com/PuvtB8q.png

Am I doing something wrong?