You could look at the color with your eyes and compare the color formed with some standards. This is a visual measurement of the color. Visual methods can give very good results when applied in an appropriate manner. However, visual measurements of color are subjective. Everyone's eyes are different.
Some people are color blind.
Colors might look different when there is different lighting. When your testing protocol requires the utmost accuracy and precision these problems must be solved. You use a colorimeter. LaMotte colorimeters were developed to make measuring color differences in water easy. The detectors in these colorimeters are very sensitive and can detect very small differences in color.