What is the rainbow method of communication in Percy Jackson novels?

There isn't a specific communication method called the "rainbow method" in the Percy Jackson novels. This phrase doesn't appear in the books, nor is there any related concept used for communication.

It's possible you're thinking of something else, perhaps a different method of communication within the series, or a fan-created term.

Could you please provide more context or details about what you remember about this "rainbow method"? This might help me understand what you're referring to and provide you with a helpful answer.