(defun color-digits-p (color)
  (save-match-data
    (string-match (rx bos
                      "#"
                      (1+ (repeat 3 3 hex-digit))
                      eos)
                  color)))

(defun widget-color-match (widget value)
  (or
   ;; I am not sure what colors to test. It might be relevant to check
   ;; all as I suggest here.
   ;;(color-defined-p val)
   (member value x-colors)
   (and (stringp value)
        (color-digits-p value))))

(defun widget-color-validate (widget)
  (let ((value (widget-value widget)))
    (unless (widget-color-match widget value)
      (widget-put widget :error (format "Invalid color: %S" value))
      widget)))

(define-widget 'color 'editable-field
  "Choose a color (with sample)."
  :format "%{%t%}: %v (%{sample%})\n"
  :size 25  ;; (length "light coldenrod yellow") = 22
  :tag "Color"
  :match 'widget-color-match
  :validate 'widget-color-validate
  :value "black"
  :complete 'widget-color-complete
  :sample-face-get 'widget-color-sample-face-get
  :notify 'widget-color-notify
  :action 'widget-color-action)

(defun custom-type-symbol-p (symbol custom-type)
  "Return t if value of symbol SYMBOL should fit CUSTOM-TYPE."
  (let ((found nil)
        (type (get symbol 'custom-type)))
    (while (and (not found) type)
      (setq found (eq type custom-type))
      (setq type (car (get type 'widget-type))))
    found))

(defun custom-type-value-p (value custom-type)
  "Return non-nil if value of VALUE fits CUSTOM-TYPE."
  (let ((widget (if (listp custom-type)
                    custom-type
                  (list custom-type))))
    (setq widget (widget-convert widget))
    ;; There are (unfortunately) two different ways to test the
    ;; values in a widget. Some widget types use both, some just one
    ;; of them. We check for both, but only use one of them here.
    (let ((match-fun (widget-get widget :match))
          (validate-fun (widget-get widget :validate)))
      ;;(setq match-fun nil)
      ;;(setq validate-fun nil)
      (widget-put widget :value value)
      ;; Fix-me: I am not sure whether widget-apply of funcall
      ;; should be used here, but I believe anyone of them can be
      ;; used. But please look into this. It might have something to
      ;; do with internal/external values for the widgets.
      (cond
       ;; Test the :match alternative first because this because this
       ;; seems most basic.
       (match-fun
        (when
            ;;(widget-apply widget :match value)
            (funcall match-fun widget value)
          t))
       (validate-fun
        (let (;;(val (widget-apply widget :validate))
              (val (funcall validate-fun widget)))
          ;; Check if :error was applied
          (when (not (widget-get val :error)) t)))
       (t
        (error
         "There is no way to check value against custom type %s"
         custom-type))))))

(defun custom-type-p (val-or-sym custom-type)
  "Return non-nil if VAL-OR-SYM fits CUSTOM-TYPE.
VAL-OR-SYM may be either a variable or a symbol. If it is a
variable then return non-nil if the value fits custom type
CUSTOM-TYPE.

If it is a symbol then return non-nil if the values this symbol's
variable can have fits CUSTOM-TYPE."
  (if (symbolp val-or-sym)
      (custom-type-symbol-p val-or-sym custom-type)
    (custom-type-value-p val-or-sym custom-type)))

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;;; Tests

;; (custom-type-p 'test-color 'color)
;; (custom-type-p 'test-color 'edit)
;; (custom-type-p 'test-color 'editable-field)
;; (custom-type-p test-color 'color)
;; (get 'test-color 'custom-type)
;; (setq test-color "bla")
;; (setq test-color "black")

(defcustom test-color "black"
  "color test"
  :type 'color)

(defun max-color-length()
  (let ((len 0)
        (longest ""))
    (mapc (lambda (color)
            (when (< len (length color))
              (setq len (length color))
              (setq longest color)))
          x-colors)
    (cons len longest)))
;; (max-color-length)