Well, a good angle grinder would take that righ...
Oh wait, might not wanna go that route...
If acetone won't take it out, I wouldn't try anything more aggressive. What about color filling black over the white?
If it is coloring that someone added try some Acetone and a tooth brush followed up buy a protective coating of Remoil(or your preference) to prevent corrosion. I have a Citadel. It wasn't painted. So hopefully somene added it with model paint or nail polish and Acetone should remove it. Anything more aggressive then acetone will probably affect the parkerizing.
Nail polish remover.aka acetone.
acetone
If acetone won't take it out, I wouldn't try anything more aggressive. What about color filling black over the white?
Never mind.
Thought my name was being called.