Preheat oven to 180 degrees C. Grease an oblong baking dish.
Place the beef mince into a frying pan over medium heat then brown the meat, breaking it apart into crumbles as it cooks, about 8 minutes. Drain excess fat. Stir in the taco seasoning, water, chilli powder, cooked rice, salt, garlic salt, black pepper and half the tomato puree; mix until thoroughly combined. Bring to a boil, reduce heat to low and simmer 20 minutes.
Meanwhile, cut the capsicums in half lengthwise then remove stems, membranes, cores and seeds. Place a steamer insert into a large saucepan and fill with water to just below the bottom of the steamer. Cover then bring the water to a boil over high heat. Place the capsicums into the steamer insert, cover the saucepan then steam until just tender, 3 to 5 minutes.
Place the steamed capsicums into the prepared baking dish then fill lightly with the meat filling. Press 1 cube of Colby cheese into the centre of the filling in each capsicum then spoon the tomato puree over the capsicums. Cover the dish with aluminium foil.
Bake in the preheated oven until the capsicums are tender and the filling is hot, 25 to 30 minutes.