The Rain Collector is a building which collects Water when it is raining. It can be constructed after researching Rain Collector.
It cannot convert Acid Rain into useable Water. Despite the red-hued rain of Vain being understandably dubious, it is harmless and can be collected whereas places like the Deadlands or The Unwanted Zone's Acid Rain is hazardous and uncollectable. The Litmus test should be "Is the colored/acid rain deadly to an unprotected Human?" If no, harvestable. If yes, uncollectable.
Rain Collectors are the only water source which can be built in Player-Owned Buildings in Town. This building is fully automated and does not require any workers nor use any power.