If the 'muddiness' disappears after a while it is certainly air. Usually this is white in appearance tho'... However, if the water is of certain chemical/mineral content this can lead to reactions around the bazillions of tiny air bubbles; the resulting compound is coloured giving the water the appearance of containing pigment or dirt.
Air ingress into water systems is a problem; even under pressure a pipe can draw air in if the flow-rate is high; rather like a carby on a petrol engine. If left, this air usually collects into visible bubbles, and larger pockets which can be a ball-ache in terms of hammering and air-locks.
Sometimes, on pumped systems where there is a sudden pressure drop - beyond for instance a partially closed valve or on the leeward side of a pump impeller, you get 'cavitation'. This is where the dissolved air in water forms bubbles ( like when you open a fizzy drink ), these then burst against surfaces and in doing so dislodge tiny amounts of the metal they are made of. In time this will lead to leaks and wear - that are often confused with chemical corrosion - and are a costly headache for water utilities!
So, air...possibly! Also could be dissolved pigmentation from the groundwater supply itself. Do you live in a peaty area for instance? Are your kettles stained a ghastly brown colour where they are constantly immersed in water?
Don't know if any of this helped or not!!
Deer mee...mi speeling is hopless twonight
[This message has been edited by uksparky (edited 09-30-2004).]