What is Iris-messaging? How does it work? Chapter 15

Annebeth uses a water spray gun to make a rainbow because Iris sometimes provides messages in the rainbow for half-bloods. It is sort of like texting, except with rainbows. Iris messages a picture of Luke with a bronze sword. All of them are standing on the porch of the Big House. Luke is asking how they are and says they have issues with campers. There is supposed to be a standoff between Zeus and Poseidon and this tension is reflected in the camp.