Even a black Sharpie will work. You may have to tilt the adapter so the viewing angle is just so then the writing will appear. I started doing that years ago when the adapters had been breeding like bunnies and all my markings are still legible. It works even if not as well as a silver/white marking pen and nowhere near as well as using a label maker but even black markings have allowed me to identify adapters I would otherwise have not known what they were meant to charge. Considering after a few years the black Sharpie tends to turn brown thus making the writing a little easier to read any marker should work, just depends how prominant you want the writing to be.