This is a beautiful new dataset! The four colors represent four different phytoplankton types. The large phytoplankton (red, yellow) have the fastest growth rates and dominate where there are lots of nutrients to nourish them at high latitudes and near the equator where nutrient-rich water upwells from the deep ocean. On the other hand, the small phytoplankton (green, blue) are more competitive where nutrients are very low. Read more here: http://sos.noaa.gov/Datasets/dataset.php?id=630 | NOAA S