Combine the potatoes, cucumber, onion, celery seeds, salt and pepper. Toss lightly.
Remove the egg yolks and combine with the sour cream, mayonnaise, vinegar and mustard. Add to the potato mix and toss very lightly.
Let stand 15 minutes before serving.
Garnish with chives or parsley or paprika or strips of pimiento - indeed, with anything of distinctive flavor and bright color.