Pure Water Fountain

The Witch Doctor carries every variant from the moment he moves in, with no progression gate on any of them. A pre-hardmode base can have Hallowed water in it long before the Hallow exists in the world, and he stocks both world evils even though your world only contains one. The exceptions are world generated: Drunk worlds put Icy and Desert fountains on Floating Islands, and Celebration Mk 10 islands carry Hallowed ones.

The color change is the obvious job, but a fountain also satisfies recipes that want water nearby, the same way a sink does. That is the practical reason to park one in a crafting room. No pond, no shoreline, no bucket runs.

You can flip a fountain by hand or wire it to a switch, a timer or a pressure plate. Wiring is the tidy option in a large build, since it lets one lever recolor a whole room without walking to the wall each time.

A fountain reaches far past its own sprite, wide enough that two placed anywhere in the same base will overlap. Where the areas cross, the fountain further to the right takes the contested water. If you want two colors in one build, separate them by more than feels necessary and walk the seam before you commit to a layout.

The recolor follows things that only loosely count as water: Wet Bomb spray, Magic Water Dropper particles, wet rocket bursts, the conch and Shellphone ocean effect, Eyebrella rain, Cute Fishron's eyes, and your own hair under Biome Hair Dye. It ignores most of the things you would expect it to catch, including Water Bolt, the Aqua Scepter, the Wet debuff, waterfall blocks, the Water Candle, bottled water and any critter jar with water in it.

None of this touches fishing, spawn rates or any other system. Pick the variant that suits the room you are building, not the biome you happen to be standing in.