The colour of under water light is determined by the changes in the spectral composition of the light as a result of the optically active components in the water. The main components are phytoplankton, non-living particulate matter (tripton), the coloured dissolved organic substances or aquatic humus (CDOM) and water itself. The spectral changes in the underwater light are caused by absorption- and scattering processes. Absorption causes light to disappear whereas scattering changes the direction of the light.
The water colour can be registered as a reflection spectrum, this is the ratio between the outgoing light and the incident light as a function of the wave length. This reflection spectrum can be reconstructed from measurements, for example remote sensing from satellites, aircraft or using a field spectrometer, or can be simulated using an optical model such as UITZICHT.