What it says on the tin, basically. If you use the materials overlay and set it to metals, they stay background grey. I guess this is to show the element of the gas of liquid in it, but I think this specific overlay should show the materials the mesh tiles are made of, at least. In the example below, the mesh tiles are in a vacuum.
Screenshot for clarity, there's mesh tiles to the right of the volcano (top two steel, bottom one wolframite):
Make something out of metal (ore), use materials overlay (F4), select "metal".