When only one tap runs brown, cloudy, or gritty, the first job is to find out whether the water coming into the house has changed or whether something at that fixture is shedding debris. Those two causes call for different actions, and buying a treatment system addresses only one of them.
Check the other taps first
Go to every other tap in the home, including the kitchen, each bathroom sink, the tub, and any outdoor hose bib. Run each one cold for a few minutes and note the color. If the other taps run clear, the water entering the house is probably fine, and the trouble sits in the piping that feeds that one tap or in the tap itself. If several taps show the same color, the cause is more likely upstream of the fixture, and you should move to the supply questions below.
Look at the aerator and the cartridge
Unscrew the aerator, the small screened tip on the end of the spout, and inspect it. Sediment, flakes, or grit caught in the screen point to debris in the line, which often follows work on the plumbing. Rinse it and reinstall it. If the color returns quickly with a clean aerator, the source is more likely the valve or the supply line serving that tap. On a single-handle faucet with a cartridge, a worn or dirty cartridge can also release particles. Check the manufacturer's instructions for removal before you open anything.
Compare hot and cold at that tap
Run the cold side alone, then the hot side alone. If only hot runs discolored, the water heater or the hot supply line is the likely source, and sediment collected in the tank can be part of the picture. If only cold is affected at that tap, check whether the supply line serving it has been disturbed. If both are affected, the cause could be the line feeding the whole group of fixtures.
Watch what happens over time
Note whether the water clears after a steady run and whether the color returns after the tap has sat unused for a while. Debris that clears and stays clear usually came from a one-time disturbance in the line. Color that comes back each time the tap is used after a rest, or that shows up on a regular schedule, suggests a steady source such as corrosion inside older piping. That is a plumbing question, not a filter question.
What to ask before anyone opens a wall
If the aerator check and the hot-and-cold comparison do not settle it, ask a licensed plumber three things: which lines feed that tap, what pipe material is known to be in use, and whether the shutoff at that fixture works. Ask your water utility whether it has any work scheduled or recorded in your area that could stir up sediment, and whether it will test the water at your meter. A plumber can tell you whether the piping itself is failing. A treatment provider can only address water quality, so ask any provider directly whether a filter would fix a pipe problem, and get the answer in writing.
Before you buy
A whole-house filter will not stop color that comes from corrosion inside the house, and it may not stop debris from a single line. If the problem is confined to one tap, a treatment purchase is usually premature. Keep a short log with the tap, the color, how long you ran it, and what you changed. That record is what a plumber, utility, or provider will ask for first, and it keeps you from paying for equipment that solves a different problem. For how to judge provider claims, see the review methodology.