This work presents the development and evaluation of a low-cost, multi-wavelength optical sensor for real-time monitoring of the SoC and total vanadium concentration in VRFBs. Using a led light source and a commercial off-the-shelf (COTS) multi-wavelength sensor with six compatible spectral channels ranging from 445 to 630 nm, we demonstrate that accurate measurements can be achieved without relying on expensive full-spectrum UV-Vis spectroscopy systems. Using spectral deconvolution and a calibration based on 66 carefully prepared samples, the sensor demonstrated its ability to quantify the SoC and the total vanadium concentration with root mean square errors of 2.4% and 70 mM, respectively. These results confirm that discrete spectral sensors can effectively capture the essential optical features of vanadium electrolytes absorbance spectra, offering a reliable alternative to traditional full-spectral methods. The proposed system is characterized by its costeffectiveness and practical design. With a total assembly cost of less than € 150, it provides a significant reduction in the financial and technical barriers typically associated with SoC monitoring in VRFBs. This makes the technology more accessible and scalable, offering a promising solution for energy storage systems where cost-effectiveness is critical.