Some color in the water can be from metal ions themselves if they are concentrated enough, but you generally won't see that in pools. Usually, if you see a color, it means something has precipitated and is like you describe, a pigment which is a suspension and yes, that can often be filtered out. It may need a clarifier or flocculant to aid in removal.
However, due to the risk of staining of pool surfaces, if one sees either the start of staining or water discoloration that is clear (which is usually metals, while cloudy green is usually algae), especially if this is brought on by a recent rise in pH and/or TA, then one usually tries lowering the pH first. And yes, if there is color and the metals test says zero, try lowering the pH to see if this reduces the discoloration. If it does, then you've very likely got metals and lowering the pH redissolves the metal compounds back into metal ions in water so a subsequent metal test will usually report metals. At that point, you can use a metal sequestrant, though in the case of iron you may need to do an ascorbic acid treatment since metal sequestrants only remove the ferrous form (Fe2+) of iron while chlorine will usually oxidize this to the ferric form (Fe3+). Ascorbic acid is a reducing agent that will convert ferric to ferrous thereby allowing the metal sequestrant to bind to the iron.
The more typical scenario is that metal ions build up from use of a copper-based algicide or pouring acid through the skimmer stripping copper from a heat exchanger or having lots of iron in fill water (usually well water). In these cases, there can be staining or discoloration even at normal pH and TA levels because the metal ion level is so high. In this situation, one usually measures metals in the water (i.e. it's not zero) since both metal ions and metal compound precipitate can be present simultaneously (enough to be both measured and seen, respectively).
Even when using a metal sequestrant, you have only temporarily solved the problem. Chlorine will break down metal sequestrants over time (it does so more slowly with HEDP compared to EDTA) so additional metal sequestrant needs to be added periodically. Dilution of the water with metal-free fill water is needed to physically remove the metals from the water.
It is too bad that the precipitation of metal so often leads to staining of pool surfaces (especially plaster) since it would be fairly easy and convenient to simply raise the pH and/or TA to force precipitation and then filter out the metal, but this is a very risky proposition in most cases.
Richard
