Nail polish works just about everywhere too, lasts for years (unlike electrical tape, which is excellent temporarily), and is trivial to find. It can take a couple layers, but if you just want to dim it that can be a good thing.
(edit: though I have no idea if it'll let infrared through like this post covers. I luckily haven't had any devices sharing IR windows like that)
(edit: though I have no idea if it'll let infrared through like this post covers. I luckily haven't had any devices sharing IR windows like that)