2 Base Robo (vs. Zerg)
From Liquipedia Starcraft 2 Wiki
This is the typical mid-game build most Protoss players opt for against Zerg. It provides great balance against the main Zerg mid-game units (Roach, Hydralisk) while allowing for establishment of map control.
Once you have secured your natural expansion, this build allows for continuous output of Colossus and Warpgate units, which can be a devastating combination against the standard Hydra/Zergling composition. If presented with mass Roaches, the player can easily transition away from Colossi to Immortals, which demonstrates the flexibility of the build.
 Build Order Notes
As this is a mid-game push strategy, the specificity of an initial build order is not as crucial. This build can be transitioned to from either the 1 Gate Core or 2 Gate Rush. Here are some pointers that will greatly increase the build's effectiveness.
- As a general rule, you might safely sustain continuous production from one Robotics Facility and three Warpgates per mining base. This means you should be able to build 2 Robos and 6 Warpgates and maintain consistent production cycles from both. However doing so severely limits your options as you might be gas starved; a more conservative setup is 1 Robo and 5-6 Warpgates, once your natural is saturated.
- Extended Thermal Lances is nothing short of imperative, as Colossi without it are within attack range of Hydralisks. Having the extra +3 range allows them to stay back and attack while being out of harm's way.
- As a general rule you should be keeping up with upgrades, but in this build it can really make all the difference.
- Zealots with +1 Weapons can two-shot a Zergling and can beat Hydralisks that are not effectively micro'd away.
- Colossi with +2 Weapons can one-shot every Zergling (without armor upgrades) in their linear path; a handful of Colossi can melt away swarms of lings in just a few seconds.
- A single Immortal with +1 Weapons can beat 4 Roaches single-handedly.
- Sentries are crucial in this match-up, and fairly viable to incorporate. Both the Colossus and Immortal cost more minerals than gas, and the Zealots you build require no gas at all. In addition, proper use of Force Fields will keep Hydras at bay while allowing your Colossi several uncontested shots.
- Use proxy Pylons to reinforce pushes and keep the pressure up. Building several Pylons at different areas in the map allow for constant widespread pressure and reinforcement; these "spotter" Pylons also increase your map awareness, giving you valuable information on the opponent's movement and actions.
This is not a "lean" build - the major unit (Colossus) is extremely expensive. This means that every Colossus counts, and you should be focusing your micro on keeping them out of enemy range. Move the weakest Colossus back, and maintain a high Zealot count since they will be tanking damage.
When Zerg players see Colossi they often go for Roaches and Infestors. Be aware of this, and adjust your composition accordingly. Add Stalkers and Immortals to deal with Roaches, and the moment you spot an Infestor try to focus it down before it can mind control a Colossus.
As this is a mid-game build, the assumption is that you have done good scouting and know whether your opponent is going for Hydra/Ling, Hydra/Roach, or possibly even Mutalisks. If you suspect a Muta-heavy composition, this is not the best build. Colossi are extremely vulnerable to Mutas, and your anti-air unit count will be quite low. This build is meant to beat ground armies.
The build really shines against the more common Speedling/Hydra mid-game composition. Using Force Fields to block off the enemy and Zealots to tank, Colossi can sit back and melt away large groups of units without worry. In addition, their long range makes retreat easy as well.
 Mass Roach
Colossi are in no way useless against high Roach counts, but they have 145HP and natural armor, which makes them significantly more robust than Hydras. More importantly, the Immortal is a natural hard-counter to the Roach, and is cheaper than the Colossus anyway. If you see mass Roach, cut Colossi for Immortals and Zealots for Stalkers. Keep your Sentry count high, as Guardian Shield reduces Roach damage by 12.5%.
If you spot the Spire early, substitute one of the Robotics Facilities for a Stargate. Phoenixes deal effectively with Mutalisks, but keep in mind that very few players go purely Mutas. Typically once they've sunk the majority of their minerals/gas into Mutalisk production, they supplement with Speedlings. Keep your Zealot and Colossus count reasonable, but play off what your opponent gives you. More Mutas means more Phoenixes. More Zerglings means more ground army.
Most Zerg players who spot this build will begin to incorporate Corruptors into their Hydra/Zergling ball. This presents a serious problem for this build because of the value of each Colossus. If you scout the Spire being built, it is sometimes possible to end the game with a push just prior to the Spire finishing and the Zerg has Larvae saved up for Corruptors. If the Spire is already up, or if the Zerg defenses are too tight, it is advisable to either transition into High Templar, or incorporate blink Stalkers into your army composition. In army clashes, have your Sentries and Stalkers attack the Corruptors first, as it is imperative that as few Colossi get taken down as possible, since they are your primary damage dealers.
Some Zerg players employ Infestors in their anti-Colossus mix, so be on the lookout. If they are allowed to mind-control your Colossi, Infestors will single-handedly turn the tide against you. Use Force Fields to keep them out of range and focus your fire on them.
If one of your Colossi is mind-controlled, try to snipe the Infestor controlling it. If that seems impossible, take out the Colossus. While losing one of your own Colossus is unfortunate and most players will hesitate to do so, it is a much better outcome than allowing the Colossus to kill your forces.
You can always introduce some High Templar into the mix for feedback on the Infestors.
 Pro features
Maps with lots of high-ground and cliffs that you can exploit for Colossus positioning and drops.
Also great are maps with several narrow chokepoints and alleys, which can cause your opponent's units to clump tightly. With a good concave, you can maximize the effect of the Colossus splash and inflict heavy casualties.
 Con features
Any map that is fairly level and wide open.