color and pixel types + loop optimization

Folks,

I would like to optimize the loop below and use the for .. <type> =
form to retrieve color and pixel. This seems to require me to specify
the type for color and pixel, etc. I figure that the color type is
color::rgb (:rgb) but what is the type returned by
gp:image-access-pixel?

    Thanks, Joel

P.S.
		  (loop for x fixnum from 0 below width
		     for tx fixnum from ty by 4 
		     for tr fixnum = tx 
		     for tg fixnum = (1+ tr)
		     for tb fixnum = (1+ tg)
		     for ta fixnum = (1+ tb) do
		       (let* ((pixel (gp:image-access-pixel image-access x y))
			      (color (color:unconvert-color port pixel))
			      (mask-pixel (gp:image-access-pixel mask-access x y))
			      (mask-color (color:unconvert-color port mask-pixel))
			      )

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