The scales of the fish are colorful.

Meaning: This means that the fish has shiny, flat pieces covering its body that have many different colors. It's like saying, The shiny, flat pieces that cover the fish's body have different colors like red, blue, green, and yellow.

Choice of Words: Scales are small, flat, hard plates that cover the body of a fish. Colorful means having many different colors. Of the fish shows that the scales belong to the fish. When you put them together, "The scales of the fish are colorful" describes the scales of a fish that have different colors.