In reply to minus273degrees:
I got a Tendon rope marker from Leeds Wall shop. I've used it to mark two ropes so far. One's a Mammut Phoenix 8mm iceline type rope and the other is my day to day 50m 8.5mm cragging rope - forgotten the make. Both are predominantly blue. The marker is black. Both ropes show some tiny pinkish tinges at the edge of the markings I made.
I'm happy that no substantial deterioration of the rope or it's properties will ensue. I'm also happy to have a middle mark at last.