For plants, pigment content has shown to be a remarkably consistent biomarker across chemicals with different modes of action. In this study, we evaluated the use of pigment content as endpoint in binary mixture toxicity studies compared to three growth endpoints on the floating plant Lemna minor. Six binary combinations of six herbicides with different mode of action were used. Data were tested against both the concentration addition (CA) and independent action (IA) reference models. For CA, two statistical approaches were used. The study showed that for some herbicide combinations the mixture toxicity measured on pigment content did not reflect the results measured on plant population growth, emphasizing the importance of measuring growth in parallel with biomarkers. CA explained the data just as well as IA, and the two different statistical models used to test the data in relation to CA showed very similar results.