In nature, most males are the colorful one and the females are not as colorful. (not as pretty) Why? Because the males want to attract the females and mate. If you look at a peacock the ones with the "eye" feathers are the male (the blue ones) the brown non-colorful ones are the female peacocks. So my guess is that the colorful fish are the males and the non-colorful ones are females. But don't take my advice, I don't know I'm right! it's just a guess!
I hope it helped (even though it was a guess!
angelfish
One example of a brightly colored fish that starts with an "o" is the Ocellaris Clownfish, also known as the "false percula clownfish." These fish have vibrant orange bodies with white stripes.
Although I'm not sure what you mean by "deep" body, Betta fish, or Siamese Fighting Fish, or Rumble Fish (if you're S.E. Hinton) are thin, brightly-colored (domestically bred males tend to be more colorful), and have long, wavy fins.
Some different species of birds
brightly colored males:)
Because they are trying to warn predators that they are poisiness
To attract pollinators.
Cocks were created with brightly colored feathers to make them noticeable to females who may be attracted to the colors.
A brightly colored blanket or cloak is called a serape. It is worn in Mexico, Central America and South America.
i do not know sorry
The tropics
There are thousands of possibilities.