It took me most of the week to get a good video of a Queen Triggerfish. Notice how it changes color as it swims by. Since many of my favorite fish can change color, I became interested in learning how it happens.
The New World Encyclopedia tells us,
Chromatophore is a pigment-containing and light-reflecting cell, found in various invertebrate and cold-blooded vertebrate animals, that can help bring about changes in color or brightness in the organism. Among animals with such cells are various crustaceans, cephalopods, amphibians, fish, and reptiles. Through aggregation or dispersion of the pigment and reorientation of reflective plates in single cells (in most animals) or through the muscular movement of complex chromatophore organs (in cephalopods), the color or brightness of the entire organism or part of the organism can change, which is useful for camouflage, signaling, temperature regulation, and protection from radiation.https://www.newworldencyclopedia.org/entry/Chromatophore
It looks like this Queen Triggerfish was changing color to blend with the surroundings for camouflage.