I don't know if this has been discussed before, but I think it would be a neat idea to have stockpiles look like whatever stone/wood/block etc that makes up the stockpile. Similar to how buildings and stuff works, have a primary, secondary and tertiary visual options.
Another option can be if it is made up of one material, have that be the main texture/color, but if it has more than one type just revert it back to the normal stockpile look.