A colour film which produces a finished colour print 50 seconds after the shutter of the camera has clicked was announced by the Polaroid Corporation yesterday. The film, to be called Polacolor, will be on sale in the United States later this month, and is expected to be available in this country towards the middle of the year. This remarkable advance in colour photography has been forecast for some time. Polacolor is the product of 15 yearsÕ research in the corporationÕs laboratories in Cambridge, Massachusetts. It can be used in the majority of Polaroid Land cameras which currently produce ÒinstantÓ black-and-white prints Ñ the colour prints will not even need the coating which is required with black-and-white pictures. The whole process takes place within the camera.