I was going to recommend what Chris said. I have seen Patrick Ball play many times, and I noticed that when he uses markers to mark the strings he only marks them on the tops, above where he plays. He can see the color there, but doesn’t touch the colored part when he plays.
I heard back from Patrick about what pens he uses. Here is what he said:
“Opaque paint pens work well. Sharpie makes them, as well as Permapaque. And yes, as Kari sagely points out, everything wears off. But, in this as elsewhere in life, I try to avoid confrontation. So, I only color the upper part of the string. My fingers never touch the color.”
He also asked me to wish you good luck with your new harp.
Some players don’t color their strings at all, and find their way by knowing their top and bottom strings, tracking vibrating strings, finding particular intervals (half–steps, gaps or perhaps two strings tuned identically in the middle of the harp’s range), or even gauging from a mark in the grain of the wood next to a particular string.
However, most harpers do use color to at least some extent. If you have trouble seeing your strings and have tried the obvious course of re–applying their color, there are a couple of things you can try. You can use a brighter color, say pink instead of red for your C’s or turquoise for the F strings. You can take a fine piece of colored thread and tie a bit of it high up on the string or down near the soundboard. Some people color their string shoes or tuning pins especially those who are new and taking harp lessons. Paint may work better than using markers (try a model paint from a hobby shop). If you decide to use paint or permanent markers, do be very careful not mark the wood of your harp.