rTrue colour is caused by dissolved compounds in water. It can be natural or artificial. Dissolved and suspended solids (together) cause apparent colour.
Other harmless constituents of water such as tannins, iron and manganese, may also be present and impart colour. The HI-727 reports the colour of water from 0-500 PCU (Platinum Cobalt Units).
Also available: Hi-727-11 Calibration Set.