Colour value is normally not measured in seawater. A collateral variable is "yellowing substance" (also mentioned "gilvin").
Yellowing substance is a measure of humic substances and degradation products from carbohydrates in the seawater. It is most common in coastal areas, especially estuaries. There is thus often a linear correlation between the concentration of yellowing matter and salinity, although the former is not a function of the latter. Measurement of yellowing substance can be used to determine the spread and mixing of water masses. The concentration of yellowing substance is given in the unit "absorbance per metre".
Methods of determination:
Method 1: The colour value can be determined either through comparison with a standard solution of platinum chloride ions, or with standard colour discs in a comparator. Yellowing substances can be measured in situ with a transparency meter, which is lowered into the sea or in a lab with a spectrophotometer.
Method 2: Measurement with optical instrument
In order to be able to standardise tests, absorbance has started to be used as an alternative to traditional colour value and is included in the EN-ISO-norm. Absorbance is in that case measured in a spectrophotometer.
The colour value of the sample water is also affected by suspended substances, such as flocks of humic substances, algae, detritus, particulate organic and inorganic matter, etc.
True colour and apparent colour
True colour is caused by loose substances in the water. Apparent colour is the colour of unfiltered water. It is measured when you want to determine the quality of natural water. Only apparent colour is measured at visual inspections.