This research documents the aims and the impact of a teaching experiment concerning colour phenomena. This teaching experiment is designed in order to make students consider not only the spectral composition of light but also its intensity, and to consider the absorption of light by a pigment as relative, instead of as total or zero. Eight teaching interviews conducted with third-year university students were recorded, transcribed and coded. Their analysis suggests that two "anchoring cognitive reactions" were likely to facilitate students' learning in a forthcoming sequence on this theme. It also makes it possible to evaluate the importance of a strong obstacle, that is the interpretation of absorption/transmission curves, or the multiplicative aspect of subtractive synthesis. Finally, the students' comments about their feelings at the end of the interview introduce a brief discussion about the benefits and/or frustration in terms of intellectual satisfaction. (Contains 2 figures, 6 tables, and 2 notes.)