Unless they've changed in the past 6 months, Colossi are ridiculously overpowered, a lot like flying siege tanks without the setup time or friendly fire.
But it's necessary for them to be like that, because Protoss gateway armies simply can't handle MMM or tier 2.5 zerg armies without them when the game is at that stage. Once it gets into the late game, Gateway is fine due to having 20-40 warpgates sitting around and the money in reserve to instantly remax on zealot/archon and attack-move to win or produce stalkers as needed, but before that, Protoss needs a superpowered unit that can kill the piles of marines or roaches (or whatever it is that zerg use these days, back when I played 12 minute max roaches the "in" thing) or they'll just get steamrolled.
And it's due to Warpgates being a bad decision choice. So much in SC2 is all wonky simply due to warpgates completely negating defender's advantage.