So there is no strategy that leads to HT + Colossus, and really any Protoss who runs that build is kind of shooting themselves in the foot. Let me explain:
The Colossus is an excellent unit, it provides AoE ground damage while maintaining a significant range. Unfortunately, these beasts are balanced by some severe drawbacks: Long tech time and heavy gas investment. In addition to have to go Gateway->Core->Robo->Bay, the Colossus becomes most effective after it receives the Thermal Lance upgrade. This is an extremely long tech lead up. Frequently players will go 3 Gate pressure just to provide the time necessary to build up to Colossus and reach critical number.
The High Templar is an excellent unit, it provides AoE ground damage while maintaining a significant range. Unfortunately, these beasts are balanced by some severe draw backs: Long tech time and heavy gas investment. In addition to have to go Gateway->Core->Archives->Counsel, the High Templar only really becomes effective after it receives the Psionic Storm upgrade. This is an extremely long tech lead up. Frequently players will go 3 Gate pressure into Robo and only transition to High Templar as a late game move because of the time necessary to build up to High Templar and reach critical number.
Notice how similar those two paragraphs were? That's probably because I copy and pasted most of it. The Colossus and the High Templar serve the same role. There is no real reason to go Colossus + High Templar. Both are heavy gas sinks. Instead, it's more frequent for Protoss players to go Colossus and then transition into High Templar. The important part about this transition is that you have to transition as well.
For a Colossus you'll want to utilize Corruptors, Roaches, or Ultralisks to eat through the opposing army quickly and take down these money units. By comparison High Templar are more vulnerable to Mutalisk, Roaches, or Brood Lords. Units which can either survive the damage or snipe the High Templar. The important point here is that if your opponent is transitioning you need to transition as well.
Its also important to note that there is no Zerg unit which really counters the High Templar. Unlike the Colossus which is vulnerable to a specialized unit like the Corruptor, or relies on other units as a shield (thus making it vulnerable to the Ultralisk), there is no real weakness for the High Templar. They are more balanced by the extremely high resource investment you need to get to them. Usually if you get to the point where your opponent has High Templar you have a Tier 3 army of your own you can pressure him with. If you find yourself against early High Templar then chances are your opponent made a large sacrifice to get them early and you missed a crucial opportunity.
I'd never say High Templar are unbeatable, as Protoss you really need to be on about 3 Base before you can transition safely to HT. However they are extremely good against a number of core Zerg units: Zerglings, Hydralisks, Infestors, Mutalisk. So be very careful when approaching them. They require a lot of finesse.
The last thing I will mention is a quote from Jeff "iNcontroL" Robinson:
High Templar are kind of like that building in Age of Empires, you know the one where you have to build it and then hang on to it for 20 minute then you automatically win the game... Wonders, that's the one.