In this work, we show an optoelectronic system based on a plastic optical fiber (POF), which can detect changes in the RGB composition in liquid solutions. This system consists of a white light (LED) with 3 W of power, and 20 centimeters of POF with cladding in the U-configuration,where 0.5 cm is without cladding that allows having contact with the rest of the solution. Also, the arrangement of photodiodes allows us to capture the intensity of light that comes from the fiber. Furthermore, we tested different solutions where we observed changes in the RGB color composition, using different concentrations of water with alcohol. These results shall allow us to design in the future a prototype for measuring the refractive index in different liquid solutions.