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@fotoclickix: "If I make multiple copies of a photo (cmd+T) and color-code each copy, they are not added to the group of other color-coded photos, but remain in the uncoded rest of the session. ...if I group all the images with the color "red" within a folder in your version 1 (i.e., sort them by label color), then Lightroom Classic "overlooks" an image that, although marked red, is only Lightroom's copy of another, unlabeled image."
When you make a virtual copy (Cmd T in German), LR places the copy at the bottom of a stack with the original. The stacks are indicated with a number in the upper-left corner of the top photo in the stack and the two vertical lines on the top and bottom photos in the stack:
When in the folder or collection containing the stack, LR always presents the photos in the stack grouped together, regardless of the chosen sort order. LR uses the attributes of the top photo in the stack when choosing how to sort it with respect to other photos.
So if you label the virtual copy at the bottom of a stack Red and then sort Library Grid view by Label color, the stack will appear next to photos that have the same label color as the top photo, not next to Red photos.
You have a couple options to get that Red virtual copy sorted next to other photos labeled Red:
1. Remove the virtual copy from the stack by right-clicking it and doing Stacking > Remove From Stack.
2. Drag the virtual copy to be the top photo in its stack.