Very high risk you will damage tiles. Also you cannot regrout over a grouted area. Best is probably finding a product that stains. Or even easier allowing a bit of daily use to let it turn naturally grey.

Thanks Asv was thinking that But looked at a few videos on the Internet that made it look easy. Mmmm...Never thought of staining. Do you know of any particular product? Daily use is turning it greyish but only in the areas that get more use. V patchy.

Maybe create a test set. set a few tiles with grout on a board wait a few days and see how well you do cutting the tile out.

I was thinking of a product like this

http://www.aquamix.com.au/products/grout-colorant/

If you have not used epoxy grout then you can try spreading dirty water to the places that are used infrequently. One bucket of dirty water that is carefully spilled to pour along the joints might hurry on the process.

You need this product by Universal, photo below!

Thanks NYC Have you used this before? Were you pleased with it?

Thank you