I get that color code in the CSS Live panel no matter where I am in the HTML file. Oddly, my File Preview panel is black and white. I guess that's following my Dark theme, though Edit > Preferences > Editor Settings > Color Scheme is set to pyte-light. I swear that yesterday the whole Editor UI was pyte-light. I'm not sure if I updated to Calibre 8.10 in between the possibly-hallucinated difference or not (I'm on Windows 11).
EDIT: the following site seems to be showing that color matching the background color of the code(?) panel:
https://rgb.to/rgb/221,221,221