The picture looks like black and white to me.
I think i would try a "solid" black (or navy);or white first . Maybe alternate black ( navy) and white backgrounds for a checkerboard background.
try laying the plates on different colored backgrounds. Sometimes one has to learn what definitely does not work before getting to something that does work.
Offhand, i think dark brown, dark green, or eggplant would not look good.
Oops! I thought the plates were still 'loose". What are they applique on?
Mayhe something with a microdot print? it would read as a solid.
Maybe try various shades of pink or blue? maybe a gray - but the shade of grade wouod matter.