If you're using Quick Look from the finder (highlight file and press the space bar), you can use ctrl + 1 - 7 to tag the selected files a specific color.

With Quick Look, you can still use the up and down arrow keys to go through your photos one by one, which might mean you don't need the Preview app (where this doesn't work)

Tested on Mojave, source: https://support.apple.com/en-gb/guide/mac-help/mchlp15236/mac

If you're using Quick Look from the Finder (highlight file and press the Space), you can use Ctrl+1 - 7 to tag the selected files a specific color.

With Quick Look, you can still use the up and down arrow keys to go through your photos one by one, which might mean you don't need the Preview app (where this doesn't work)

Tested on Mojave, source