Yellow water across the whole house points at the supply or the well. Yellow water from one faucet is a different problem with a much shorter list of causes, and all of them are within a few feet of the tap. Working through them takes minutes and usually ends without any treatment equipment being bought.
Confirm it really is one tap
Run the cold water at two or three fixtures well apart in the house and compare. If they all run yellow, the cause is upstream and this is a supply or well question rather than a fixture one. If only one does, the cause is local. Also check whether it clears after running for a minute, which points at water standing in that branch, and whether it happens on the hot side only, which points at the water heater rather than the tap.
The aerator and the fixture itself
Unscrew the aerator and look. Rust coloured grit trapped in the screen is common on older galvanized pipework and after any plumbing work, and it will discolour the first water through every time the tap is opened. Clean or replace it. Then look at the fixture: an old tap with corroding internal parts, or a flexible connector hose degrading inside, both discolour water at one outlet only. A filter fitted at that tap and long overdue is another local cause worth ruling out.
Standing water in a little used branch
A tap that is rarely used has water sitting in the pipe serving it, sometimes for weeks, in contact with whatever that pipe is made of. In galvanized or old iron pipework that produces exactly this yellow first draw, clearing after a minute. The remedy is to use the tap or to flush it periodically, and where a branch is genuinely redundant, capping it back at the main run removes a dead leg that is doing nothing useful for water quality anywhere in the house.
When it is the water after all
If several taps are affected, follow the standard sequence: let a glass stand to see whether it clears from the top down, which indicates oxidized iron or disturbed sediment, or stays uniformly coloured, which indicates dissolved iron, manganese or tannins. Penn State Extension notes iron and manganese carry secondary aesthetic standards with recommended maximums of 0.3 mg/L for iron and 0.05 mg/L for manganese. Our faucet filter guide covers point of use options once the cause is known.
Questions people ask about yellow water from faucet
Why is only one tap running yellow?
The cause is local: a rusty aerator screen, a corroding fixture or connector, or water standing in a little used branch.
It clears after a minute. What does that mean?
Water has been standing in the pipe serving that tap. Flushing it periodically, or removing a redundant dead leg, is the answer.
What if only the hot side is yellow?
That points at the water heater rather than the tap or the supply, typically a corroding tank or a spent anode rod.
When should I suspect the supply?
When several taps are affected. Then let a glass stand: clearing from the top down means particles, staying coloured means dissolved.