I have collected over the past few years 3 round boiled sweet tins; one ea. for brass / cast iron / aluminium filings that on occasions I have used to mix with 2 part epoxy resin to fill blow holes & the like. Cast iron filings work particularly well leaving a barely discernable difference. As mentioned brass can vary in shade so are visible in most cases. Larger brass chippings can be used to temper / blue small items… see Clickspring's site where he shows an example of bluing tiny screws. I have used Aluminium too but that shows a difference on shading too, more like a 'chilled' spot as in Cast iron,again due to different grades, they all do the job & if you're painting afterwards, who's to know?
George.