I've just bought something a friend of mine suggested for touching up black metal items on the bike (note I haven't tried it yet as I'm waiting for it to arrive). He used to be an Armourer repairing guns and he used it on trigger guards and barrels, it's called Birchwood Casey Flat Black. You can either buy a pen (which I did) or a solution, be careful as one type is NOT suitable for Aluminium. You can get a flat black or a gloss black https://www.ebay.co.uk/itm/Birchwood-Casey-Flat-Black-Super-Black-Metal-Scratch-Repair-Touch-Up-Pen/251312617996?ssPageName=STRK:MEBIDX:IT&_trksid=p2057872.m2749.l2649 https://www.ebay.co.uk/p/1408784891
I always used a Stabilo too lol ! In bright sunshine ( not that we get a lot of that) it can sometimes look purple ... but its a tenner cheaper than the stuff on Ebay ... and I am from Yorkshire
Tried it out and it’s pretty good, slightly bluer finish on my black anodising but better that a shiny mark