Stop by a real sporting goods store or gun shop and pick up a small jar of "super blue" by Birchwood Casey which should be next to all the gun lubes. Its a blue container about the size of a pill bottle for about $5 or so. It's for touching up blued steel but it'll tarnish copper and brass too. Methodology is buy the finest scrotchbrite pad you can find and apply the blue on the pad, then -lightly- scuff the light up. I suggest using gloves unless you like having stinky black fingertips
It'll turn dark fairly fast when you remove surface oxidation when the solution has some fresh brass to react on. If it seems to slow down, apply a little more juice. If you just swab it on, it'll take much longer, or not even work properly. After it turns really dark or even black, wash it off with water to get all the bluing fluid off. Don't worry if it isn't even and splotchy.
Then take some really fine scotchbrite, steel wool, or polishing compound, and gently work it on the light. This will slowly lighten the tarnish. Keep working on it until you get to the level of patina you want and manage to smooth out uneveness. Then I just lightly oil the light and call it a day. Of course, test on a scrap piece of brass first (something around the house or a hidden spot on the light?) to get comfortable with the process and remove the o-rings. I don't think bluing fluid is good on rubber.