Advances in fiber technology have enabled the integration of plastic optical fibers (POFs) with light-emitting diodes (LEDs) for the development of simple, cost-effective, and sensitive sensing systems. The aim of this letter was to develop a low-cost device for measuring adulteration in liquids. In this letter, we present a low-cost optoelectronic system based on a U-shaped POF intrinsic sensor to measure the changes in RGB composition of light transmitted as an evanescent wave through liquid solutions (tequila-water mixtures). The system comprises a 3 W white LED as the light source, a 20-cm-long U-shaped POF, and an RGB photodiode (PD) array that detects changes in the RGB composition of light propagating through the POF. An uncoated 5-mm-long U-shaped POF is in contact with the analyte solution during the measurements. The system is tested using tequila-water mixtures of different concentrations. The RGB PD response is recorded digitally, which improves the efficiency and accuracy of the measurement while reducing the sensitivity of the sensor to external disturbances. The results indicate a system resolution of 0.3 wt% with respect to the tequila concentration, as well as red, green, and blue color sensitivities of 0.1, 0.6, and 0.4 8-bits/wt%, respectively. These findings can contribute to the development of an automated system that efficiently measures adulteration in liquids such as distilled spirits.