I hope I'm not telling you what you already know Karen, but it seems that the sanding you refer to was to remove the wax and give the paint a key.

Can I suggest you do this, if possible use wet 'n' dry on a small area of marezzo, to see if you can bring the colour back, use plenty of water and try not to scrub too hard.

If the marezzo was made in the traditional manner the veins of colour should have soaked deep into the plaster/keenes cement. So if your careful you could restore the colour.

Hope this helps

Ed