I need to swap a colour with transparency
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.