The Forel-Ule scale is a method to estimate the color of bodies of water. The scale provides a visual estimate of the color of a body of water, and it is used in limnology and oceanography with the aim of measuring the water's transparency and classifying its biological activity, dissolved substances, and suspended sediments.
The classic Forel-Ule Scale uses a set of liquid vials of multiple colors. Together, the liquid vials represent a standardized color palette created by using a set of small transparent glass tubes containing colored water by adding different concentrations of stable inorganic salts. By mixing different chemicals (distilled water, ammonia, copper sulphate, potassium-chromate and cobalt-sulphate) a standard color scale is produced in a set of numbered vials (1-21). The set of vials is then compared with the color of the water body. The result is a color index for the water body which gives an indication of the transparency of the water and thus helps to classify overall biological activity. The color graduations correspond to open sea and lake water colors, as they appear to an observer ashore or on board a vessel. The method is often used in conjunction with the Secchi disk submerged to half the Secchi depth, so that the color can be judged against a white background.
A set of color lighting filters against a white background can also be used as a Forel-Ule scale, called a Modern FU plastic scale. High-quality lighting filters of many colors are combined with one another to create the 21 colors of the traditional Forel-Ule Scale when viewed against a white background, such as white plexiglass. 
The method was developed by François-Alphonse Forel and was three years later extended with greenish brown to dark brown colors by the German limnologist Wilhelm Ule. The Forel Ule scale was a simple but adequate scale to classify the color of rivers, lakes, seas and oceans. The Forel-Ule scale observations, along with temperature, salinity, bathymetry, and Secchi depth, are some of the oldest oceanographic parameters dating back to the 1800s.
Role in citizen science
In the Netherlands, a project called the Citizen’s observatory for Coast and Ocean Optical Monitoring (Citclops) project has begun crowdsourcing water color measurements from citizen scientists. Citizen scientists estimate the color of the water with the Forel-Ule scale using a smartphone app called “Eye on water.”   
- Citizen science
- Color of water
- Munsell color system
- Ocean color
- Pt/Co scale
- Secchi disk
- Water quality
- Wernand, M. R.; Woerd, H. J. van der (27 April 2010). "Spectral analysis of the Forel-Ule ocean colour comparator scale". Journal of the European Optical Society: Rapid Publications. European Optical Society. 5. Bibcode:2010JEOS....5E0014W. doi:10.2971/jeos.2010.10014s. ISSN 1990-2573.
- Novoa, S.; Wernand, M. R.; Woerd, H. J. van der (21 July 2014). "The modern Forel-Ule scale: a 'do-it-yourself' colour comparator for water monitoring". Journal of the European Optical Society: Rapid Publications. European Optical Society. 9. doi:10.2971/jeos.2014.14025. ISSN 1990-2573.
- Pitarch, Jaime (1 September 2020). "A Review of Secchi's Contribution to Marine Optics and the Foundation of Secchi Disk Science". Oceanography. The Oceanography Society. 33 (3). doi:10.5670/oceanog.2020.301. ISSN 1042-8275. S2CID 221972701.
- Garaba, Shungudzemwoyo; Friedrichs, Anna; Voß, Daniela; Zielinski, Oliver (18 December 2015). "Classifying Natural Waters with the Forel-Ule Colour Index System: Results, Applications, Correlations and Crowdsourcing". International Journal of Environmental Research and Public Health. MDPI AG. 12 (12): 16096–16109. doi:10.3390/ijerph121215044. ISSN 1660-4601. PMC 4690980. PMID 26694444.
- "Measuring water colour". Citclops. Retrieved 6 January 2022.
- "Water colour - Education". Eyeonwater.org. Retrieved 6 January 2022.