TL;DR: always clear the caches of a TYPO3 system after messing with processed files.

As it turns out, image files can contain color profiles.

As it also turns out, those image files don’t look to good if you strip those profiles.

By default, TYPO3 strips all embedded color profiles of an image if it needs to create processed files (e.g. thumbnails) using ImageMagick to make those generated files a bit smaller.

After adjusting the colorspace setting, I deleted all processed files via the Install Tool, but did not clear the frontend caches. This way, TYPO3 tried to reference all the old generated image files but could not find them, resulting in empty images shown in the frontend.

Takeaway from this: at least clear the frontend caches after messing in some way with the generated image files to ensure proper display.