Disposable cup for preparing, applying and preserving a paint to be set up on a paint spray gun characterized in that it includes on one of its end faces a closable vent device (7) forming a vent valve with valve duct (22) and valve plug (25) which allows when opened, to let the air pass for occupying the inner volume that empties and when closed, to close liquid-tightly the opening (6) of air passage to form a container for the preparation of the paint and then to form a pot for the preservation of the leftover paint. This invention is of interest to manufacturers of paint spray guns.