Lightroom Classic "Sortieren"

There is a metadata field EXIF Label, which you can see in the EXIF and IPTC tagset of the Metadata panel:

The menu command View > Sort > Label Text sorts by the contents of that field.

You can put any text you want in that field. But LR's color commands also set that field. When you assign the color red by typing 6, LR goes to the current color label set (Metadata > Color Label Set) and looks up the label text currently assigned to the color red, and it stores that text in the EXIF Label field:

For the Bridge Default set, typing 6 to assign the color red will store "Select" in the EXIF Label field.