At some point during my educational process (still ongoing) I learned that 1. water is colorless, and 2. the color we do see is a reflection of the color of what is most prevalent around it. Blue, grey, and green, tend to be most common, as might be expected. For some reason, though, there are those bodies of liquid that don't adhere to that rule. Take the little reservoir behind the dam in San Dimas Canyon for example. Its shallow depths always seem to be a dark inky black. The conditions of the day do not seem to matter - it can be grey and overcast with filtered light, or it can be bright and sunny - the water will be black all the same. One interesting side effect of that inkiness - when conditions are right, the surface becomes quite a good mirror.