I have seen an actual brass treatment that colors the brass. Can't remember exactly what it is called, though.
Now, you can also invest in a brass catcher. For AR15's, a number of these are available that strap right onto the rifle and cover the ejection port.
Or, you can get a bit inventive....
I would recommend some closely woven nylon mesh cloth. For the sake of expedience, you can sew some two-sided velcro tape at relatively close intervals around the perimeter of the cloth. Make the piece of cloth BIG, as well.
Now, obtain something to make the frame work from. Again, for expedience, I would recommend PVC pipe, in a small diameter. Make lengths with couplers so that you can simply slide it together. Connect the mesh to the PVC with the velcro straps. You can make a stand to put by a shooting bench, and make it big enough to extend below the bench, and overlapping to the front and rear.
You're in business! You now have a good brass catcher, and you can blast away with a glad heart and a clean conscience. You can be very versatile with this design as well.
Make it long and shallow, and you have a good catcher for prone shooting.
You can get enough to catch brass while shooting from the standing position as well. It would have to be pretty big, though.