Background: Front-of-pack (FOP) nutrition labelling has received growing attention from public health authorities, as a tool to promote healthier diets in the population. Recently, the French Health law introduced the principle of implementing a FOP nutrition labelling system. A scientific proposal has put forward the 5-Colour Nutrition Label (5-CNL), a five-category colour-coded summary system supported by research suggesting that it is well perceived and understood in the population. Our objective was to investigate the impact of the 5-CNL on the nutritional quality of purchases in experimental supermarkets. Methods: Participants (n = 901) were recruited using quota sampling between September and December 2015 and evenly distributed in three experimental conditions: 1) control situation; 2) Application of the 5-CNL on all food products in three specific sections: breakfast cereals, sweet biscuits and appetizers; 3) introduction of the 5-CNL accompanied by consumer information on use and understanding of the label. Main outcome was the nutritional quality of the shopping cart in the three sections combined, measured using the United Kingdom Food Standards Agency nutrient profiling system (FSA score). Results: Significantly higher mean nutritional quality of the purchased items per section were observed for the sweet biscuits category in the intervention combining the label + communication (overall FSA score 21.01 vs. 20.23, P = 0.02). No significant effects were observed for the general mean over the three sections combined or other food categories. The results observed on purchase may be related to the high level of recall, self-reported and objective understanding of the label that were observed in the intervention groups as they are pre-requisites for use of a label in purchasing situations. Conclusion: These results suggest that the 5-CNL FOP nutrition label may have a limited impact on purchases, leading to healthier food choices in some food categories such as sweet biscuits.