Explanation:

- The passage describes a journey through various islands, focusing on the natural beauty found on a particular peaceful island.

- The blank in the sentence refers to the description of birds.

- Option 1: "colours" is a noun and does not fit well as it doesn't describe birds directly.

- Option 2: "colour" is a noun but in singular form, which is less suitable in this context.

- Option 3: "colouring" implies the action or result of adding color, which doesn't suit an adjective requirement.

- Option 4: "coloured" is an adjective that describes the birds as having bright colors, fitting the sentence context perfectly.