Randomly assign a colour to every mine spot when it's generated. Ore from the mine that doesn't turn into metal turns into stones of this colour. Pavement is same colour as stones used to pave it. Plain old boulders give boring grey stones.
This would encourage some form of trade because everybody likes pretty colours and would beautify the game a lot. And it requires almost zero effort - just apply a colour mask over existing graphics. It'd also add just one variable to stones and pavement, which shouldn't be that hard on the server, especially if you restrict it to a small number of colours.
Main use for this is in the thread title.