(defclass boring-gradient (design)
  ())

(defmethod draw-design (medium (design boring-gradient) &rest drawing-options)
  (apply #'invoke-with-drawing-options medium
         (lambda (medium)
           (loop for x from 0 below 400 by 1 do
                 (draw-line* medium 0 x 1000 x :ink (make-rgb-color
                                                     (/ (- 400 x) 400)
                                                     0 (/ x 400)))))
         drawing-options))

(defun foo ()
  (draw-rectangle* *standard-output* 0 0 400 400
                   :ink (make-instance 'boring-gradient)
                   :clipping-region (region-difference
                                     (clim:make-ellipse* 200 200 100 0 0 100)
                                     (clim:make-ellipse* 200 200 50 0 0 50))))