Fountain pen ink does not work, it is water based and not thick enough to change the color of the light.

I wonder if pen ink might hold up better? Let me think about it. I may rip apart one of my pens, see how the ink looks against a piece of glass [may be too thick or too thin].

If it looks like it might work I will give that a try.

Since I was "in there" I applied another Sharpie coating, this time from a different pen, albeit I don't think the other Sharpie was defective.

Sidenote: The old ink cleaned right up with some denatured alcohol dabbed on to a tissue that I wrapped around a small screwdriver.