I think the schemes laid down for the various hive fleets are kind of like a default setting. When a fleet arrives on a given planet they will be their normal hive fleet specific colours, but the longer the battle goes on the more you would start seeing creatures with colours more reflective of the environment the fighting is taking place in, as killed and injured nids are reabsorbed and recycled. When the planet is completely stripped, almost all of the nids would be absorbed, and fresher ones created in the default setting in preparation for the next victim.