Sorry for all the questions. My mind keeps coming up with issues. What do darts represent before and after being used? I really don't know how to roleplay asking for ammunition or policing brass because I'm not sure what they are.
The idea is that it's somewhat magnetic waste-metal that doesn't have to be particularly well cared for in order to be flung down the barrel of the gun.
"brass" is likely a historical holdover rather than a comment on actual metal content. Other than that, we've generally treated it as metal slugs fired by railgun or coilgun tech, Potentially heated in the chamber to reform to useful shape as needed. The orange color was often justified as oxidized iron. Can probably justify the blue as a cobalt alloy.