||If the grid is being shown as a grid, it has a legend. Set appropriate legend colors to transparent. Otherwise, if it is being shown as an image, it might have a color table. Set some of its colors to transparent.
No technique will work for hardcopy, including pdf files: there is a well-known, long-established bug causing transparent colors to print as white. You can still trick AV into doing what you want by converting the tiff to a grid and then manipulating the two images with the map calculator. It's involved, but it works.