The question of permanent inks is still relevant in the fountain pen world — and that’s why you should be careful with Noodlers’ inks… I do love them all, the marketing, the provenance, etc — they are beautiful inks and Polar Brown is probably one of the nicest brown inks on the planet (much nicer than Pelikan’s Smoky Quartz)… but they’re very hard to clean and if you love Demonstrators like I do… well, you shouldn’t use them on Demos unless you try them first on lesser $$$ pens.
e.g. after using Polar Brown in a clear pen, expect the barrel/CC to be brownish or interact with the transparent colour of the barrel from that point on. (I’m thinking the TWSBIs, for instance)