This paper presents an analysis on the implementation of an ICT-based activity in a physics lesson about optics at grade 8 (lower secondary school). This activity aims at introducing the additive synthesis of colors with the help of a simulator. The analysed activity has been elaborated within a research-development group associating secondary school teachers and a science education researcher. The teachers followed in this study are informed both of the conceptual objectives and of the underlying hypotheses related to modelling process. The simulation can be used as a reference experience all along the lesson. It allows students to compare their previous ideas to the simulation results and to the real experiment results. This comparison can be used to explicit the modelling process underlying the presented simulation.