[475] How is "not percipient of form internally, one sees forms externally" a liberation (vimokkha)?
Here someone does not attend to the blue sign (nimitta) internally, in himself; he does not obtain the perception (saññā) of blue. He directs the mind (citta) to the blue sign externally; he obtains the perception of blue. He makes that sign well grasped, bears it well in mind, defines it well. Having made that sign well grasped, having borne it well in mind, having defined it well, he cultivates it, develops it, and makes much of it. It occurs to him thus: "Not percipient of form internally, this external [sign] is form"—he is percipient of form.
Here someone does not attend to the yellow sign … the red sign … the white sign internally, in himself; he does not obtain the perception of white. He directs the mind to the white sign externally; he obtains the perception of white. He makes that sign well grasped, bears it well in mind, defines it well. Having made that sign well grasped, having borne it well in mind, having defined it well, he cultivates it, develops it, and makes much of it. It occurs to him thus: "Internally there is no form; this external [sign] is form"—he is percipient of form. Thus "not percipient of form internally, one sees forms externally" is a liberation.
English translation from the Pali (Siam Raṭṭha edition) — under review, not for citation.