Although it is impossible to find water in its pure state naturally, if you take pure water and observe it through a long transparent column, a tinge of blue will appear. This tint of colour comes from the fact that H20 molecules absorb the red end of the light spectrum. In fact, the bluish colour of water is the only known example of a natural colour caused by vibrational transition.
In short, even though water is blue in nature, when we talk about the colour of water, it is usually in reference to its opacity and the contaminants visible to the naked eye found in it. Although some contaminants may not be visible to the naked eye, when the concentration of these is high enough, the colour of the water may be affected.
Presented differently, water colour is affected by two types of contaminants: dissolved solids (TDS) and suspended solids (TSS). Although dissolved solids have an impact on the colour of water, it is mostly due to suspended solids that colour is seen. For example, the presence of colloidal matter will have a very important impact on the colour.
Since "colour" of water is not a recognized water parameter, turbidity is generally used. Turbidity is a water parameter that is measured using nephalometric units (NTU). To identify the level of turbidity of water, we calculate, thanks to special equipment, the percentage of light that can pass through it.
Anyway, if you would like to learn more, here are some articles that might interest you:
- What is water turbidity
- What is the alkalinity of water
- What is water conductivity
- What are total dissolved solids (TDS)
- What are suspended solids (SS)