datastore.get(key, function(err, entity) {
  entity.color = 'red'

  datastore.save({
    key: entity[datastore.KEY],
    data: entity
  }, function(err) {})
})

datastore.get(key, function(err, entity) {
  entity.color = 'red'
  datastore.save(entity, function(err) {})
})