Yeah nearest neighbour was my first thought, but I noticed that the clusters were more spread out in some directions than other. For instance the black cluster

Yeah nearest neighbour was my first thought, but I noticed that the clusters were more spread out in some directions than other. For instance the black cluster is elongated in the saturation dimension but quite tight in H and V. I wanted to capture these directions of variance and I think the GMM does this. Put differently, I think the clusters are better approximated by ellipsoids than spheres. But yep nearest neighbour probably would have worked fine too.