I've got Indian stone flags as coping on my pond. It was lovely black when I did it and now it's faded light grey. I asked at the local builders yard (think red leader sent me) and they said there was a special product. So I looked it up,,,,129+vat a tin[:0] fair enough if your doing a patio but for the top of a wall??
If you get some and have any left over I'll buy it off you[:T]