Global change is predicted to have numerous consequences on ocean properties, notably on ‘ocean color’. For phytoplankton cells, changes in ocean color are perceived as modifications of their underwater light niches that can trigger competition between species potentially resulting in dramatic changes in community composition. To tackle this question, EFFICACY focuses on the Synechococcus, the second most abundant oceanic phytoplankton organism, and the most diversified in terms of pigmentation. It has recently been shown that chromatic acclimaters (CA4), i.e. cells capable to change their pigment content to match the dominant light color (either blue or green), were the most abundant Synechococcus pigment type in the ocean. EFFICACY studies the ecological importance and fitness advantage conferred by the CA4 process. For this purpose, EFFICACY relies on a state-of-the-art cross-scale approach that combines laboratory experiments, in-situ field work and modeling.