Hall of Triumph costs 3 colorless mana to cast. When it comes into play, you choose a color and creatures of that color you control gain +1/+1. Sadly, you can't choose colorless as an option for Hall of Triumph. Despite, colorless mana getting its own mana symbol in Oath of the Gatewatch, you still can't choose colorless as an option if you need to choose a color. It's pretty straightforward. While it's limited to a single color, there are plenty of mono-colored decks that could use an anthem effect, especially Red decks that wouldn't otherwise have one.
It seemed like Hall of Triumph would become a mono-colored Commander staple. After seeing a bit of play in Standard, however, the only deck that widely adopted the Hall was Krenko, Mob Boss. Along with the Tribal anthem, Obelisk of Urd, suddenly Krenko's Goblins got a lot more imposing. Honestly, this artifact is still relatively underplayed considering the versatility of its effect. If you're running a one-color Tribal deck in Commander, you should definitely consider running a copy of Hall of Triumph.