Last thing to try from me.

If you want 15% Grey as a table cell background – set it so.
Open an image editor, I don’t know what you use or prefer, so on this occasion, we will use MS Paint for this experiment.
Open Paint, then open the PNG ‘Smaller’ file previously used.
Click on ‘Fill with Colour’ icon > Edit Colours.
Change Red value to 217
Change Green value to 217
Change Blue value to 217
OK

Now click in the white areas to change to the 15% grey that we have set.
Save As a JPEG > Accept warning.
Use this new stamp image (jpg) in a cell for a comparison.
Test print > Fingers crossed > Time for a bottle of red!



. . . . . that's a bottle of red wine, not Red Label Whisky!!