These little fish can have quite different coloration. The silvery one to the right is a common sight. The one above is a male in his courtship and spawning colors. Later, when he’s guarding the eggs, he’ll be a more silvery blue with an orange patch on the lower jaw. Juveniles can vary from silvery blue to yellow.
Many other fish also have extensive color variations, which can make it difficult identifying individuals. Oval Chromis are one the larger family of damselfishes. While their colors and patterns vary quite a bit, they all have a similar ‘look,’ which, in this case, helped narrow down what I was looking at.
In my attempts to identify what I see in the water, I use John P. Hoover’s book The Ultimate Guide to Hawaiian Reef Fishes, Sea Turtles, Dolphins, Whales, and Seals. His website is hawaiisfishes.com.