An experimental procedure is presented that was developed by fifth-year chemical engineering and industrial chemistry undergraduates at the Federal University of Rio de Janeiro doing the discipline on Experimental Organic Technology. The aim of this study was to apply the solvatochromic effect of the dye Nile Blue chloride to the characterization of biodiesel/diesel blends with different biodiesel content, using an alternative image processing analysis method involving low-cost, simple, and rapid assays. The effect of the solvatochromic dye Nile Blue chloride was monitored using the software ImageJ, which is being used currently, yielding rapid and effective responses. As an outcome, the students proposed a procedure that had potential application to colorimetric determination of biodiesel content in diesel oil. This gave students the opportunity to put the knowledge acquired in the course into practice, by dealing with issues concerning the fuels industry, making the learning process more dynamic, engaging, and effective.