Triumph hasn't really bothered with getting polarity right inside the indicators. For normal glowy light bulbs, it doesn't matter which way the power goes through them, the wire starts glowing regardless. So they just bunged in the wires and called it a day. Heck, on my Touring, all the wires going back there are black and silver, and then someone haphazardly strapped some red tape around one pair to indicate which side they go to...
I'd suggest you get a multimeter (you can buy cheapo ones for literally just a few bucks in any auto parts type store or off Amazon) and actually measure them. Take the glass off and the bulbs out of the indicators and expose the connectors.
Connect one multimeter cable to the bike frame, some exposed metal where you get a ground connection (like a bolt), and then measure which of the connectors inside the indicator actually give you zero resistance. That's your ground/negative. If the negative is in the wrong place, pull the reflector out (assuming it's like my Touring at all) and switch the cables behind the reflector around.