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How about:

(defun my-insert-color-hex (&optional arg)
  "Select a color and insert its 24-bit hexadecimal RGB format.

With prefix argument \\[universal-argument] insert the 48-bit value."
  (interactive "*P")
  (let ((buf (current-buffer)))
    (list-colors-display
     nil nil `(lambda (name)
                (interactive)
                (quit-window)
                (with-current-buffer ,buf
                  (insert (apply #'color-rgb-to-hex
                                 (nconc (color-name-to-rgb name)
                                        (unless (consp ',arg)
                                          (list (or ,arg 2)))))))))))