To send a message, one must first find or create a rainbow with Mist and then throw a golden drachma or anything with a good price into it, saying or thinking very deeply:
"Oh Iris, goddess of the Rainbow, please accept my offering."
"Oh Fleecy, do me a solid. Show me (name and place of person)."
This is usually said aloud, though it works non-verbally as well. Then the caller says the name or location of the intended recipient. The way to end an Iris Message is to wave one's hand through the Mist, disconnecting the signal. Iris Messages can also be sent to gods, such as when Percy contacted Mr. D. This is odd, because in The Titan's Curse, Percy wonders if it is even possible.
In The House of Hades, Gleeson Hedge is shown to keep, amongst other things, a stack of golden drachmas, a flashlight and a glass prism for making rainbows which leaves him prepared to send a lot of Iris-messages.
In The Battle of the Labyrinth, Percy receives a "collect" Iris message, asking him to deposit drachmas in the image for each five minutes of conversation. This happens only once and the sender was dead, so it is not clear whether this is a unique event.
In The Lost Hero, Medea calls for an Iris Message with a diamond instead of a coin. This probably means that an Iris message can be activated using any object of value. Greeks use drachmas to activate Iris Messages because this was the Ancient Greek currency. It is implied in The Son of Neptune that Roman heroes do not use Iris Messages (neither Frank nor Hazel had ever heard of this method of communication) since Iris is her Greek counterpart, and by then Romans used more advanced methods, like giant eagles.
Failed Iris Messages
In The Son of Neptune, Percy tries to contact Annabeth Chase via Iris Message, but the Iris Message can't get through. It is implied that this is because Percy can't clearly remember the person he is trying to reach as Hera stole his memory and Hera herself had blocked the message.
Throughout The Trials of Apollo, Iris-messaging fails as well as all other means of demigod communication due to the interference of Triumvirate Holdings. In The Tyrant's Tomb, this is revealed to be because of the Triumvirate harnessing the powers of the soundless god Harpocrates. After he is convinced to willingly fade by Apollo, Reyna Ramírez-Arellano and Meg McCaffrey, Iris-messaging is restored along with all other methods of communication.
During their quest, Annabeth Chase and Grover Underwood use a nozzle at a car wash to establish an Iris-message to Camp Half-Blood. The two explain Iris-messaging to a confused Percy Jackson and make contact with Luke Castellan as Chiron is currently busy. The trio report on their progress before the message is broken when their time on the nozzle runs out.
After being captured by Luke on the Princess Andromeda, Percy spots a rainbow and tosses his last drachma into it before silently praying to Iris and loudly announcing Dionysus at Camp Half-Blood. Percy tricks Luke into confessing to poisoning Thalia's Pine Tree before revealing that Dionysus and all of Camp Half-Blood heard him through the Iris-message. As a result, Chiron is exonerated and Luke angrily breaks the connection. Shortly afterwards, Chiron comes to Percy, Tyson and Annabeth's rescue, explaining that Iris is an old friend of his and had allowed him to listen in on Percy's message to Camp Half-Blood.
After being cornered by Dr. Thorn the Manticore, Percy contacts Camp Half-Blood through an Iris-message, but gets Dionysus, much to his dismay. After Percy asks for his help, the god kills the manticore and saves them.
After the battle, Hermes uses an Iris-message to quickly flick through various mortal newscasts like a TV.
An amnesic Percy attempts to contact Annabeth via Iris-message without success, but is blocked by his amnesia and Hera.
Hazel Levesque contacts Reyna Ramírez-Arellano to warn her about the Giant Army, inadvertently calling her while Reyna is in the bath. It's revealed that the Romans have never heard of Iris-messaging, instead relying on other methods of communication. In this case, Iris herself gives Hazel instructions for how to make contact with the cloud nymph Fleecy giving her a "direct line." Iris also admits that Fleecy is actually the one who currently handles most Iris-messages and not the goddess herself.
After his memories begin returning, Percy tries another Iris-message without success.
When Leo attacks New Rome under the control of eidolons, Hazel tells the other members of the Seven to Iris-message her when they're safely away so that they can rendezvous. However, after Festus detects Hazel and Arion running below the ship, Frank instead shapeshifts to fly down and tell her in person instead.
It's mentioned that while Percy was missing, Annabeth had spent hundreds of golden drachmas on Iris-messages to every friendly spirit, demigod or monster that they had ever met in search of him without any luck.
When the Romans arrive in Charleston, Annabeth orders Piper to use the Argo II's galley sink to Iris-message Jason to get back to the ship quickly. After returning to the ship's deck, Piper reports that she got the message through although it was kind of fuzzy and that Jason is already on his way back.
After stealing Achelous' horn, Piper Iris-messages Leo to have the Argo II ready for a quick takeoff upon their return.
When the Seven discuss Iris-messaging Reyna Ramírez-Arellano, Hedge states that they aren't working very well currently and have horrible reception. Hazel states that they have already tried calling her without success and that some magic is interfering, possibly Gaea. It is much harder to call the Romans, causing Hazel to speculate that they are shielding themselves.
Frank Zhang later finds Hedge communicating with Mellie through another Iris-message as the Argo II arrives in Epirus. Frank notices that the coach's nightstand contains a bottle of water, a stack of golden drachmas, a flashlight and a glass prism for making rainbows, causing Frank to realize that the coach had come prepared to send a lot of Iris-messages.
Chiron reveals to Apollo and Meg McCaffrey that Iris-messaging is down. When Meg asks what Iris-messaging is, Apollo quickly explains that it's two-way visions and a method of communication overseen by the rainbow goddess. Although Apollo dismisses it as Iris just being flighty, Chiron reveals that all other forms of demigod communication are also down and that it's just one of many problems that they're facing. It's later revealed that Triumvirate Holdings are responsible.
While discussing warning Camp Jupiter about Caligula's impending attack, Calypso mentions that all forms of demigod communication are still down. As a result, Leo volunteers to go on Festus to warn them in person.
Apollo, Reyna Ramírez-Arellano and Meg McCaffrey discover that the Triumvirate is using the soundless god Harpocrates to block all demigod communications. After they convince him to willingly fade, Iris-messaging and all other methods of communication are restored.
|Percy Jackson, Grover Underwood, and Annabeth Chase||Luke (though intended for Chiron)||Reporting about their quest.||The Lightning Thief|
|Percy Jackson and Annabeth Chase||Chiron||Reporting about their quest and asking for advice.||The Sea of Monsters|
|Percy Jackson||Dionysus||So the camp can hear Luke's plans to reform Kronos.||The Sea of Monsters|
|Percy Jackson||Tyson||To talk.||The Titan's Curse|
|Percy Jackson||Sally Jackson||Telling her that he'll be coming home.||The Titan's Curse|
|Percy Jackson||Dionysus||Calling for help and to report about the Ophiotaurus.||The Titan's Curse|
|Bianca di Angelo||Percy Jackson||Showing him Nico's current situation.||The Battle of the Labyrinth (twice)|
|Eurytion||Annabeth Chase||To warn her that Nico has returned to the Labyrinth and has been communicating with King Minos.||The Battle of the Labyrinth|
|Chiron||Typhon (his location)||To show the Head Counselors the status of the gods' battle with the giant.||The Last Olympian|
|Hermes||Mortal newscasts||To check on mortal reactions to the Battle of Manhattan||The Last Olympian|
|Medea||Jane||To weaken Piper's confidence and reveal how her father was captured.||The Lost Hero|
|Thalia Grace||Jason Grace||To summon him to the Wolf House to free Hera.||The Lost Hero|
|Percy Jackson||None||Tried to call Annabeth, but it was blocked by Hera.||The Son of Neptune|
|Hazel Levesque||Reyna Ramírez-Arellano||Called her while she was taking a bath (unintentionally), Hazel warned her about the Giant Army.||The Son of Neptune|
|Percy Jackson||Tyson||To tell him to take Ella to Camp Half-Blood.||The Mark of Athena|
|Annabeth Chase||Every friendly spirit, demigod and monster she's ever met.||To search for Percy while he is missing||The Mark of Athena|
|Piper McLean||Jason Grace||To warn him to return to the Argo II fast.||The Mark of Athena|
|Piper McLean||Leo Valdez||To warn him to have the Argo II ready for a quick takeoff.||The Mark of Athena|
|Gleeson Hedge||Mellie||To check in on her while he is with the Seven.||The Mark of Athena and The House of Hades|
|Hazel Levesque||Reyna||Attempted to call her without success possibly due to interference from Gaea.||The House of Hades|
|Apollo||Diana||To summon her help in the Battle of San Francisco Bay.||The Tyrant's Tomb|