Most printing machines can only print on material up to about 450 microns thick. So to make those special very thick business cards and invitations, we print the sheets separately and glue them together.
This gives us some other tricks to use; we can combine colours so the result is red on one side, and black on the other. We can use several layers to give a coloured sandwich effect. And we can emboss on one sheet without leaving an indentation on the sheet behind.
We can do this with any material, so there is no restriction on the colours used.