I am completely unable to find out how the colours came to be inverted or how to uninvert them. However, I have achieved the result in practical terms by clicking File, Revert To, Browse All Versions. This displays a sheaf of versions rather as Time Machine does. I then clicked the arrows over at the right-hand side of the window until I found a black-text-on-white-background version. I took the precaution of exporting it under a completely different name, and all is now well. My thank to those that have tried to help.I worked happily for hours in Preview on the TIFF with the text in black and the background in white before the colours were suddenly reversed. I shall try these suggestions and report back.