An educational proposal on light spectroscopy was designed according to the theoretical framework of the Model of Educational Reconstruction (MER) by means of Design-Based Research (DBR) methods. The proposal was implemented several times according to three different approaches, focused on the Light Sources exploration (LS), the Light Emission process (LE) and the Spectra Exploration (SE), respectively. This study illustrates the results coming from a pretest–posttest aimed at measuring the students’ conceptual understanding after several implementations of the proposal, with a total number of 257 upper secondary school students (17–19 years old) in northern Italy. Some items were specific for each approach, therefore here we focus on the results related to the common aspects. The data were analyzed with qualitative methods, categorizing the student’s answer for each item, according to an iterative process. The results show that, although each approach reports some specific benefits, the LS produce a greater improvement of students’ conceptual understanding. For this approach, the results about students’ ideas of the colors’ nature and the spectra formation and interpretation are reported. Implications for teaching and further research steps are also discussed.