Tannins will generally cause a yellow, yellow/brownish tinge to the water. They come from decayed vegetation or animal matter and are carbon-based. This is significant because carbon tends to deplete oxygen and tannin-laden waters will reduce the amount of free oxygen in water. This depletion of oxygen helps explain how tannins contribute to the problem of acid water.
Tannins also prevent iron from being oxidized and precipitated (process whereby iron is changed from a dissolved form to a particle). Tannins in water are responsible for many failures of iron removal filter systems and water softeners. Sometimes organic matter will bind with heme iron (organic iron found in hemoglobin*) and will cause the iron to pass through an oxidation/precipitation process while remaining in dissolved form. The best way to remove the iron in cases such as this is to treat the iron as an organic and remove it with a tannin filter media.