I have devised a semi-aquatic sapient species known as the Hexapi who until now I thought were trichromats. However, after doing my research on the evolution of the eye, I found out that colour vision may be a disadvantage underwater. As I didn't want to give them polarised light vision, I decided to give them tetrachromacy. I know the case of Concetta Anticco, who sees the world psychedelically, just like birds. I've also known that tetrachromacy enhances colour sensitivity in shadows and dim lighting. Here's the problem, though. The Hexapi carry chromatophores on their skin.
My question - How would they reliably communicate with chromatophores? How would they look to humans?
Notes - The hexapi rarely dive below 50m (~150ft) and live in rich, shallow waters. They also sport colourful striped displays on each of their 2 side 'fins', which are of course only visible to them.