I need to swap a colour with transparency I have an image and I would like to be able to make white transparent, is this possible using Preview in Mac OSX Catalina?
Preview is designed for viewing image files, making simple image adjustments, and annotations, not for full-on image editing. You cannot swap a color with transparency in Preview.

What you are looking to do could be accomplished in paid apps like Adobe Photoshop or free and open-source apps like GIMP.

Depending on a number of different factors, like image type and compression, making white transparent in image editing software can be as simple as selecting all pixels of a single color and setting them to transparent, or as complex as manually selecting and setting pixels to transparent outside of an object's boundary.