Which sentence states a fact?

 - Of all the species of damselfish, the blue chromis is the most striking in terms of color.
 - The vitamins in a blue chromis's diet can affect how brightly the fish's colors appear.
The second sentence states a fact.
The vitamins in a blue chromis's diet can affect how brightly the fish's colors appear.
It can be proved by reading a book about tropical fish.
The first sentence states an opinion.
Of all the species of damselfish, the blue chromis is the most striking in terms of color.
Most striking shows what a person believes, thinks, or feels. Another person might have a different opinion about which fish's color is most visually impressive.