Colour challenge - Friday Blue

If we talk about blue, I think of the ocean. I came across with this photo in my album, and it always makes me smile.
I was about to take a picture of the two trumpet fish in the back, when this little damsel fish (sergeant major) came and photobombed them :-)
Sergeant majors are usually not this colour. They are generally silvery grey with yellow sheen along their upper side. However, when the males are hiding in a cave guarding their nest, they change to blue, just like it's seen on the photo.